$ cc -O2 -Wall -o dm dm.c
(no warnings)

$ kldload fuse; ./dm  (as root)
[995] opened /dev/fuse fd=3
[995] daemon loop on fd 3
[995] REQ op=26 unique=0        <- mount A FUSE_INIT
[995] REQ op=17 unique=1        <- mount A mount-time FUSE_STATFS
[996] mount(/mnt/dfa) OK
[995] REQ op=26 unique=2        <- mount B FUSE_INIT (SAME fd, no guard)
[995] REQ op=17 unique=3        <- mount B mount-time FUSE_STATFS

--- serial console ---
login: fuse_init(kldload|863): FUSE ABI 7.28
fuse_mount(dm|996): FUSE UABI 7.28
fuse_mount(dm|996): FUSE UABI 7.28
panic: assertion "fmp->rfnp->vp == NULL" failed in fuse_unmount at /usr/src/sys/vfs/fuse/fuse_vfsops.c:281
cpuid = 3
Trace beginning at frame 0xfffff8011835f748
fuse_unmount() at fuse_unmount+0x419 0xffffffff82601309 
fuse_unmount() at fuse_unmount+0x419 0xffffffff82601309 
vfs_unmount() at vfs_unmount+0x7b 0xffffffff8070d70b 
dounmount() at dounmount+0x64f 0xffffffff807019cf 
sys_unmount() at sys_unmount+0x1f2 0xffffffff80701e02 
syscall2() at syscall2+0x11e 0xffffffff80bd6a0e 
Debugger("panic")

CPU3 stopping CPUs: 0x00000037
 stopped
Stopped at      Debugger+0x7c:  movb    $0,0xbdaf09(%rip)
db>