llc: make llc_ui_sendmsg() more robust against bonding changes

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

llc: make llc_ui_sendmsg() more robust against bonding changes

syzbot was able to trick llc_ui_sendmsg(), allocating an skb with no
headroom, but subsequently trying to push 14 bytes of Ethernet header [1]

Like some others, llc_ui_sendmsg() releases the socket lock before
calling sock_alloc_send_skb().
Then it acquires it again, but does not redo all the sanity checks
that were performed.

This fix:

- Uses LL_RESERVED_SPACE() to reserve space.
- Check all conditions again after socket lock is held again.
- Do not account Ethernet header for mtu limitation.

[1]

skbuff: skb_under_panic: text:ffff800088baa334 len:1514 put:14 head:ffff0000c9c37000 data:ffff0000c9c36ff2 tail:0x5dc end:0x6c0 dev:bond0

kernel BUG at net/core/skbuff.c:193 !
Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 6875 Comm: syz-executor.0 Not tainted 6.7.0-rc8-syzkaller-00101-g0802e17d9aca-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : skb_panic net/core/skbuff.c:189 [inline]
pc : skb_under_panic+0x13c/0x140 net/core/skbuff.c:203
lr : skb_panic net/core/skbuff.c:189 [inline]
lr : skb_under_panic+0x13c/0x140 net/core/skbuff.c:203
sp : ffff800096f97000
x29: ffff800096f97010 x28: ffff80008cc8d668 x27: dfff800000000000
x26: ffff0000cb970c90 x25: 00000000000005dc x24: ffff0000c9c36ff2
x23: ffff0000c9c37000 x22: 00000000000005ea x21: 00000000000006c0
x20: 000000000000000e x19: ffff800088baa334 x18: 1fffe000368261ce
x17: ffff80008e4ed000 x16: ffff80008a8310f8 x15: 0000000000000001
x14: 1ffff00012df2d58 x13: 0000000000000000 x12: 0000000000000000
x11: 0000000000000001 x10: 0000000000ff0100 x9 : e28a51f1087e8400
x8 : e28a51f1087e8400 x7 : ffff80008028f8d0 x6 : 0000000000000000
x5 : 0000000000000001 x4 : 0000000000000001 x3 : ffff800082b78714
x2 : 0000000000000001 x1 : 0000000100000000 x0 : 0000000000000089
Call trace:
skb_panic net/core/skbuff.c:189 [inline]
skb_under_panic+0x13c/0x140 net/core/skbuff.c:203
skb_push+0xf0/0x108 net/core/skbuff.c:2451
eth_header+0x44/0x1f8 net/ethernet/eth.c:83
dev_hard_header include/linux/netdevice.h:3188 [inline]
llc_mac_hdr_init+0x110/0x17c net/llc/llc_output.c:33
llc_sap_action_send_xid_c+0x170/0x344 net/llc/llc_s_ac.c:85
llc_exec_sap_trans_actions net/llc/llc_sap.c:153 [inline]
llc_sap_next_state net/llc/llc_sap.c:182 [inline]
llc_sap_state_process+0x1ec/0x774 net/llc/llc_sap.c:209
llc_build_and_send_xid_pkt+0x12c/0x1c0 net/llc/llc_sap.c:270
llc_ui_sendmsg+0x7bc/0xb1c net/llc/af_llc.c:997
sock_sendmsg_nosec net/socket.c:730 [inline]
__sock_sendmsg net/socket.c:745 [inline]
sock_sendmsg+0x194/0x274 net/socket.c:767
splice_to_socket+0x7cc/0xd58 fs/splice.c:881
do_splice_from fs/splice.c:933 [inline]
direct_splice_actor+0xe4/0x1c0 fs/splice.c:1142
splice_direct_to_actor+0x2a0/0x7e4 fs/splice.c:1088
do_splice_direct+0x20c/0x348 fs/splice.c:1194
do_sendfile+0x4bc/0xc70 fs/read_write.c:1254
__do_sys_sendfile64 fs/read_write.c:1322 [inline]
__se_sys_sendfile64 fs/read_write.c:1308 [inline]
__arm64_sys_sendfile64+0x160/0x3b4 fs/read_write.c:1308
__invoke_syscall arch/arm64/kernel/syscall.c:37 [inline]
invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:51
el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:136
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:155
el0_svc+0x54/0x158 arch/arm64/kernel/entry-common.c:678
el0t_64_sync_handler+0x84/0xfc arch/arm64/kernel/entry-common.c:696
el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:595
Code: aa1803e6 aa1903e7 a90023f5 94792f6a (d4210000)