panic: tcp_output: mbuf chain shorter than expected: 0 + 60 + 12 - 0 != 60

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 84b9791b bridge: Fix panic if the STP root is removed
git tree: freebsd
console output: https://syzkaller.appspot.com/x/log.txt?x=17628be7200000
dashboard link: https://syzkaller.appspot.com/bug?extid=d25a5352bcdf40acdbb8
userspace arch: amd64

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d25a53...@syzkaller.appspotmail.com

panic: tcp_output: mbuf chain shorter than expected: 0 + 60 + 12 - 0 != 60
cpuid = 0
time = 3571
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x47/frame
0xfffffe00212685a0
vpanic() at vpanic+0x1e0/frame 0xfffffe0021268600
panic() at panic+0x43/frame 0xfffffe0021268660
tcp_output() at tcp_output+0x3fe2/frame 0xfffffe0021268830
tcp_usr_connect() at tcp_usr_connect+0x25c/frame 0xfffffe0021268880
soconnectat() at soconnectat+0x183/frame 0xfffffe00212688e0
kern_connectat() at kern_connectat+0x1ff/frame 0xfffffe0021268940
sys_connect() at sys_connect+0xd9/frame 0xfffffe0021268980
amd64_syscall() at amd64_syscall+0x436/frame 0xfffffe0021268ab0
fast_syscall_common() at fast_syscall_common+0x101/frame 0xfffffe0021268ab0
--- syscall (198, FreeBSD ELF64, nosys), rip = 0x412e5a, rsp =
0x7fffdffdcf38, rbp = 0x3 ---
KDB: enter: panic
[ thread pid 14964 tid 100293 ]
Stopped at kdb_enter+0x6a: movq $0,kdb_why


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