kern/45617: tmpfs KASSERT: "mp->tm_bytes_used == 0" failed

[njoly%pasteur.fr@localhost]

>Number:         45617
>Category:       kern
>Synopsis:       tmpfs KASSERT: "mp->tm_bytes_used == 0" failed
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Nov 15 15:00:01 +0000 2011
>Originator:     Nicolas Joly
>Release:        NetBSD 5.99.56
>Organization:
Institut Pasteur
>Environment:
System: NetBSD lanfeust.sis.pasteur.fr 5.99.56 NetBSD 5.99.56 (LANFEUST) #9: 
Tue Nov 15 14:52:21 CET 2011 
njoly%lanfeust.sis.pasteur.fr@localhost:/local/src/NetBSD/obj.amd64/sys/arch/amd64/compile/LANFEUST
 amd64
Architecture: x86_64
Machine: amd64
>Description:
I do use a tmpfs mount for /tmp, without size limit, on various machines
and set TMPDIR accordingly. By example for my 8GB RAM + 10GB swap amd64
main workstation:

njoly@lanfeust [~]> grep tmpfs /etc/fstab 
tmpfs                   /tmp                    tmpfs   rw
njoly@lanfeust [~]> mount -vv | grep /tmp
tmpfs on /tmp type tmpfs (local, fsid: 0xab01/0x69ab, reads: sync 0 async 0, 
writes: sync 0 async 0, [tmpfs: version=1, size_max=18446744073709551615, 
nodes_max=2147483647, root_uid=root, root_gid=wheel, root_mode=1777])

At reboot, after some system builds using build.sh, i frequently hit a KASSERT
when unmounting file systems:

unmounting file systems...Skipping crash dump on recursive panic
panic: kernel diagnostic assertion "mp->tm_bytes_used == 0" failed: file "/local
/src/NetBSD/src/sys/fs/tmpfs/tmpfs_mem.c", line 64 
cpu1: Begin traceback...
kern_assert() at netbsd:kern_assert+0x48
tmpfs_mntmem_destroy() at netbsd:tmpfs_mntmem_destroy+0x4b
tmpfs_unmount() at netbsd:tmpfs_unmount+0x9a
VFS_UNMOUNT() at netbsd:VFS_UNMOUNT+0x24
dounmount() at netbsd:dounmount+0x9c
vfs_unmountall1() at netbsd:vfs_unmountall1+0x96
cpu_reboot() at netbsd:cpu_reboot+0xff
sys_reboot() at netbsd:sys_reboot+0x7d
syscall() at netbsd:syscall+0xac
cpu1: End traceback...
rebooting...

>How-To-Repeat:

>Fix: