arm64: csum: Fix OoB access in IP checksum code for negative lengths

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

arm64: csum: Fix OoB access in IP checksum code for negative lengths

Although commit c2c24edb1d9c ("arm64: csum: Fix pathological zero-length
calls") added an early return for zero-length input, syzkaller has
popped up with an example of a _negative_ length which causes an
undefined shift and an out-of-bounds read:

| BUG: KASAN: slab-out-of-bounds in do_csum+0x44/0x254 arch/arm64/lib/csum.c:39
| Read of size 4294966928 at addr ffff0000d7ac0170 by task syz-executor412/5975
|
| CPU: 0 PID: 5975 Comm: syz-executor412 Not tainted 6.4.0-rc4-syzkaller-g908f31f2a05b #0
| Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/25/2023
| Call trace:
| dump_backtrace+0x1b8/0x1e4 arch/arm64/kernel/stacktrace.c:233
| show_stack+0x2c/0x44 arch/arm64/kernel/stacktrace.c:240
| __dump_stack lib/dump_stack.c:88 [inline]
| dump_stack_lvl+0xd0/0x124 lib/dump_stack.c:106
| print_address_description mm/kasan/report.c:351 [inline]
| print_report+0x174/0x514 mm/kasan/report.c:462
| kasan_report+0xd4/0x130 mm/kasan/report.c:572
| kasan_check_range+0x264/0x2a4 mm/kasan/generic.c:187
| __kasan_check_read+0x20/0x30 mm/kasan/shadow.c:31
| do_csum+0x44/0x254 arch/arm64/lib/csum.c:39
| csum_partial+0x30/0x58 lib/checksum.c:128
| gso_make_checksum include/linux/skbuff.h:4928 [inline]
| __udp_gso_segment+0xaf4/0x1bc4 net/ipv4/udp_offload.c:332
| udp6_ufo_fragment+0x540/0xca0 net/ipv6/udp_offload.c:47
| ipv6_gso_segment+0x5cc/0x1760 net/ipv6/ip6_offload.c:119
| skb_mac_gso_segment+0x2b4/0x5b0 net/core/gro.c:141
| __skb_gso_segment+0x250/0x3d0 net/core/dev.c:3401
| skb_gso_segment include/linux/netdevice.h:4859 [inline]
| validate_xmit_skb+0x364/0xdbc net/core/dev.c:3659
| validate_xmit_skb_list+0x94/0x130 net/core/dev.c:3709
| sch_direct_xmit+0xe8/0x548 net/sched/sch_generic.c:327
| __dev_xmit_skb net/core/dev.c:3805 [inline]
| __dev_queue_xmit+0x147c/0x3318 net/core/dev.c:4210
| dev_queue_xmit include/linux/netdevice.h:3085 [inline]
| packet_xmit+0x6c/0x318 net/packet/af_packet.c:276
| packet_snd net/packet/af_packet.c:3081 [inline]
| packet_sendmsg+0x376c/0x4c98 net/packet/af_packet.c:3113
| sock_sendmsg_nosec net/socket.c:724 [inline]
| sock_sendmsg net/socket.c:747 [inline]
| __sys_sendto+0x3b4/0x538 net/socket.c:2144

Extend the early return to reject negative lengths as well, aligning our
implementation with the generic code in lib/checksum.c