panic: vm_page_free_prep: freeing mapped page ADDR

Title	Repro	Cause bisect	Fix bisect	Count	Last	Reported
panic: vm_object_vndeallocate: bad object reference count	C			974	2020d	2021d

panic: vm_page_free_prep: freeing mapped page 0xfffff8007f235000
cpuid = 1
time = 1557808054
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x47/frame 0xfffffe001faef270
vpanic() at vpanic+0x1e0/frame 0xfffffe001faef2d0
panic() at panic+0x43/frame 0xfffffe001faef330
vm_page_free_prep() at vm_page_free_prep+0x341/frame 0xfffffe001faef360
vm_page_free_toq() at vm_page_free_toq+0x1b/frame 0xfffffe001faef390
vm_object_page_remove() at vm_object_page_remove+0x375/frame 0xfffffe001faef3f0
vnode_pager_setsize() at vnode_pager_setsize+0x107/frame 0xfffffe001faef440
vtruncbuf() at vtruncbuf+0x50a/frame 0xfffffe001faef4a0
ffs_truncate() at ffs_truncate+0x16b5/frame 0xfffffe001faef690
ufs_setattr() at ufs_setattr+0x918/frame 0xfffffe001faef730
VOP_SETATTR_APV() at VOP_SETATTR_APV+0xc2/frame 0xfffffe001faef760
kern_truncate() at kern_truncate+0x289/frame 0xfffffe001faef980
amd64_syscall() at amd64_syscall+0x436/frame 0xfffffe001faefab0
fast_syscall_common() at fast_syscall_common+0x101/frame 0xfffffe001faefab0
--- syscall (198, FreeBSD ELF64, nosys), rip = 0x41309a, rsp = 0x7fffdfffdf38, rbp = 0x2 ---
KDB: enter: panic
[ thread pid 1178 tid 100565 ]
Stopped at      kdb_enter+0x6a: movq    $0,kdb_why