i386 vnode-related panic after upgrading from 6.99.49 to 7.99.1

["Paul Pedaal" <ppedaal%mail.ee@localhost>]

An i386 (Pentium 3, 640MB RAM, many hard drives - 4 ATA, 7 SCSI, 1 USB  
card reader) panics often after doing an upgrade to latest -current from  
6.99.49 (not sure about date)

When doing any significant file IO, the system will crash sooner or later  
(depending on kern.maxvnodes setting). Occurs with both ATA and SCSI  
drives no matter if the FS is mounted from a wedge or a partition. No  
errors when doing IO on the block devices themselves.

Console output with DDB (transcribed from a blurry CRT):

vrelel: missing VOP_CLOSE(): vnode @ 0xc201a4e0, flags  
(0x100030<MPSAFE,LOCKSWORK,CLEAN,CHANGING>)
        tag VT_NON(0), type VBLK(3), usecount 1, writecount 0, holdcount 0
        freelisthd 0x0, mount 0xc1c2d000, data 0x0 lock 0xc201a584
panic: kernel diagnostic assertion "sn->sn_opencnt == 0" failed: file  
"/usr/src/sys/miscfsspecfs/spec_vnops.c", line 432
fatal breakpoint trap in supervisor mode
trap type 1 code 0 eip c027b754 cs 0 eflags 246 cr2 bb9c6000 ilevel 0 esp  
d9601edc
curlwp 0xc1c5daa0 pid 0 lid 9 lowest kstack 0xd95ff2c0
Stopped in pid 0,0 (system) at netbsd:breakpoint+0x4: popl    xebp
db{0}> bt
breakpoint(c0cf518a,c0f2ae80,c0c3e6d4,d9601ef8,c2015970,c201a4e0,c201a4f4,d9601eec,c09f154b,c0c3e6d4)  
at netbsd:breakpoint+0x4
vpanic(c0c3e6d4,d9601ef8,d9601f18,c0016ab0,c0c3e6d4,c0c3e0a0,c0cd8c1e,c0cd89c0,1b0,c1fe0940)  
at netbsd:vpanic+0x121
kern_assert(c0c3e6d4,c0c3e8a0,c0cd8c1e,c0cd89a0,1b0,c1fe0940,c281a4e0,0,c28fa4f4,d9601f40)  
at netbsd:kern_assert+0x23
spec_node_destroy(c281a4e0,0,0,0,c2813000,2,c1c5daa0,c281a4e0,c0ec47c0,0)  
at netbsd:spec_node_destroy+0x1f1
vrelel(c0ec4800,2,0,c0eaf5b0,c1c5daa0,0,cac5daa0,c0968d2f,c1c5daa0,d9681f8c)  
at
netbsd:vrelel+0x486
cleanvnode(c0ec4800,c0ec4800,64,0,0,c8100321,c1c51aa0,1870000,187a000,0) at
netbsd:clanvnode+0xfb
vdrain_thread(c1c5daa0,1870000,187a000,0,c8198304,0,0,0,0,0) at
netbsd:vdrain_thread+0x5c