panic: vm_page_swapqueue: page ADDR is unmanaged

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 14aef6df truss: decode sysctl names.
git tree: freebsd
console output: https://syzkaller.appspot.com/x/log.txt?x=1040d155600000
dashboard link: https://syzkaller.appspot.com/bug?extid=22dde2eb9103e5562969

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+22dde2...@syzkaller.appspotmail.com

panic: vm_page_swapqueue: page 0xfffffe00007a7c00 is unmanaged
cpuid = 0
time = 1569014753
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x47/frame
0xfffffe0019fdf510
vpanic() at vpanic+0x1e0/frame 0xfffffe0019fdf570
panic() at panic+0x43/frame 0xfffffe0019fdf5d0
vm_page_swapqueue() at vm_page_swapqueue+0x2b6/frame 0xfffffe0019fdf630
vm_pageout_worker() at vm_pageout_worker+0x1963/frame 0xfffffe0019fdfa20
vm_pageout() at vm_pageout+0x20e/frame 0xfffffe0019fdfa60
fork_exit() at fork_exit+0xb0/frame 0xfffffe0019fdfab0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe0019fdfab0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
KDB: enter: panic
[ thread pid 17 tid 100061 ]
Stopped at kdb_enter+0x6a: movq $0,kdb_why


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[Mark Johnston]

#syz fix: Fix a race in vm_page_swapqueue().