ila: do not generate empty messages in ila_xlat_nl_cmd_get_mapping()

In the Linux kernel, the following vulnerability has been resolved:

ila: do not generate empty messages in ila_xlat_nl_cmd_get_mapping()

ila_xlat_nl_cmd_get_mapping() generates an empty skb,
triggerring a recent sanity check [1].

Instead, return an error code, so that user space
can get it.

[1]
skb_assert_len
WARNING: CPU: 0 PID: 5923 at include/linux/skbuff.h:2527 skb_assert_len include/linux/skbuff.h:2527 [inline]
WARNING: CPU: 0 PID: 5923 at include/linux/skbuff.h:2527 __dev_queue_xmit+0x1bc0/0x3488 net/core/dev.c:4156
Modules linked in:
CPU: 0 PID: 5923 Comm: syz-executor269 Not tainted 6.2.0-syzkaller-18300-g2ebd1fbb946d #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/21/2023
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : skb_assert_len include/linux/skbuff.h:2527 [inline]
pc : __dev_queue_xmit+0x1bc0/0x3488 net/core/dev.c:4156
lr : skb_assert_len include/linux/skbuff.h:2527 [inline]
lr : __dev_queue_xmit+0x1bc0/0x3488 net/core/dev.c:4156
sp : ffff80001e0d6c40
x29: ffff80001e0d6e60 x28: dfff800000000000 x27: ffff0000c86328c0
x26: dfff800000000000 x25: ffff0000c8632990 x24: ffff0000c8632a00
x23: 0000000000000000 x22: 1fffe000190c6542 x21: ffff0000c8632a10
x20: ffff0000c8632a00 x19: ffff80001856e000 x18: ffff80001e0d5fc0
x17: 0000000000000000 x16: ffff80001235d16c x15: 0000000000000000
x14: 0000000000000000 x13: 0000000000000001 x12: 0000000000000001
x11: ff80800008353a30 x10: 0000000000000000 x9 : 21567eaf25bfb600
x8 : 21567eaf25bfb600 x7 : 0000000000000001 x6 : 0000000000000001
x5 : ffff80001e0d6558 x4 : ffff800015c74760 x3 : ffff800008596744
x2 : 0000000000000001 x1 : 0000000100000000 x0 : 000000000000000e
Call trace:
skb_assert_len include/linux/skbuff.h:2527 [inline]
__dev_queue_xmit+0x1bc0/0x3488 net/core/dev.c:4156
dev_queue_xmit include/linux/netdevice.h:3033 [inline]
__netlink_deliver_tap_skb net/netlink/af_netlink.c:307 [inline]
__netlink_deliver_tap+0x45c/0x6f8 net/netlink/af_netlink.c:325
netlink_deliver_tap+0xf4/0x174 net/netlink/af_netlink.c:338
__netlink_sendskb net/netlink/af_netlink.c:1283 [inline]
netlink_sendskb+0x6c/0x154 net/netlink/af_netlink.c:1292
netlink_unicast+0x334/0x8d4 net/netlink/af_netlink.c:1380
nlmsg_unicast include/net/netlink.h:1099 [inline]
genlmsg_unicast include/net/genetlink.h:433 [inline]
genlmsg_reply include/net/genetlink.h:443 [inline]
ila_xlat_nl_cmd_get_mapping+0x620/0x7d0 net/ipv6/ila/ila_xlat.c:493
genl_family_rcv_msg_doit net/netlink/genetlink.c:968 [inline]
genl_family_rcv_msg net/netlink/genetlink.c:1048 [inline]
genl_rcv_msg+0x938/0xc1c net/netlink/genetlink.c:1065
netlink_rcv_skb+0x214/0x3c4 net/netlink/af_netlink.c:2574
genl_rcv+0x38/0x50 net/netlink/genetlink.c:1076
netlink_unicast_kernel net/netlink/af_netlink.c:1339 [inline]
netlink_unicast+0x660/0x8d4 net/netlink/af_netlink.c:1365
netlink_sendmsg+0x800/0xae0 net/netlink/af_netlink.c:1942
sock_sendmsg_nosec net/socket.c:714 [inline]
sock_sendmsg net/socket.c:734 [inline]
____sys_sendmsg+0x558/0x844 net/socket.c:2479
___sys_sendmsg net/socket.c:2533 [inline]
__sys_sendmsg+0x26c/0x33c net/socket.c:2562
__do_sys_sendmsg net/socket.c:2571 [inline]
__se_sys_sendmsg net/socket.c:2569 [inline]
__arm64_sys_sendmsg+0x80/0x94 net/socket.c:2569
__invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
invoke_syscall+0x98/0x2c0 arch/arm64/kernel/syscall.c:52
el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:142
do_el0_svc+0x64/0x198 arch/arm64/kernel/syscall.c:193
el0_svc+0x58/0x168 arch/arm64/kernel/entry-common.c:637
el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:591
irq event stamp: 136484
hardirqs last enabled at (136483): [<ffff800008350244>] __up_console_sem+0x60/0xb4 kernel/printk/printk.c:345
hardirqs last disabled at (136484): [<ffff800012358d60>] el1_dbg+0x24/0x80 arch/arm64/kernel/entry-common.c:405
softirqs last enabled at (136418): [<ffff800008020ea8>] softirq_ha
---truncated---