panic ffs_valloc dup_alloc

["Stephen M. Jones" <martians%sdf.org@localhost>]

I have NetBSD-7 deployed on many systems (the majority being amd64) and
have noticed that the busy machines (smtpd, imapd, httpd) tend to develop 
filesystem inconsistencies that get worse with time.  The inconsistencies 
usually result in a panic: ffs_valloc: dup_alloc.  

Bringing the machine up in single user and doing several fsck -f -y on 
the file system will clear these problems for a while, but they'll develop
again.

In some cases the filesystem corruption can only be corrected by a newfs.

I have crash dumps console output with messages like the following:

panic: ffs_valloc: dup alloc
dmode 8180 mode 8180 dgen 1ee653a5 gen 1ee653a5
size 0 blocks 0
ino 9439504 ipref 9439135
panic: ffs_valloc: dup alloc
cpu0: Begin traceback...
vpanic() at netbsd:vpanic+0x13c
snprintf() at netbsd:snprintf
ffs_valloc() at netbsd:ffs_valloc+0x889
ufs_makeinode() at netbsd:ufs_makeinode+0x5e
ufs_create() at netbsd:ufs_create+0x5b
VOP_CREATE() at netbsd:VOP_CREATE+0x38
vn_open() at netbsd:vn_open+0x330
do_open() at netbsd:do_open+0x111
do_sys_openat() at netbsd:do_sys_openat+0x68
sys_open() at netbsd:sys_open+0x24
syscall() at netbsd:syscall+0x9a

I've seen this problem since NetBSD-6.   Again, this is on busy servers and
not idle machines.