panic when amd unmounts nfs filesystem

[Mark Davies <mark%mcs.vuw.ac.nz@localhost>]

I'm seeing the following panic when amd tries to unmount an NFS filesystem.  
Was happening with a 4.99.60 kernel and still happens with a 4.99.62 from 
a day or two ago:

panic: kernel diagnostic assertion "np->n_dirgens == NULL" failed: 
file "/local/src/src/sys/nfs/nfs_node.c", line 319
Stopped in pid 437.1 (amd) at   netbsd:breakpoint+0x4:  popl    %ebp
db> bt
breakpoint(c0440793,ccb86b48,c0455b80,c030bb21,c1782800,101,0,c1853274,c1853274,c1782800)
 
at netbsd:breakpoint+0x4
panic(c0449a00,c040eff8,c04215f4,c042163c,13f,0,ccb86b7c,c0203604,c040eff8,c042163c)
 
at netbsd:panic+0x1b2
__kernassert(c040eff8,c042163c,13f,c04215f4,ccaf8a90,8,ccb86b9c,c0352a1f,ccb86b94,1000002)
 
at netbsd:__kernassert+0x39
nfs_reclaim(ccb86b94,1000002,0,c0404e40,c0404dc0,ccaf8a90,ccb86bdc,c034521d,ccaf8a90,0)
 
at netbsd:nfs_reclaim+0x184
VOP_RECLAIM(ccaf8a90,0,ffffffff,ccaea800,0,0,b86bdc,c0352a88,ccb86bd0,c04b1120) 
at netbsd:VOP_RECLAIM+0x1f
vclean(ccaf8a90,8,0,0,ccaf8a90,0,ccb86c2c,c02223c4,ccaf8a90,ccaf8a90) at 
netbsd:vclean+0x17d
vgone(ccaf8a90,ccaf8a90,0,c0344c1b,ccaf8a90,0,0,ccb45000,ccb45024,0) at 
netbsd:vgone+0x23
nfs_unmount(ccb45000,0,cac65ebc,0,0,0,1,0,ccaf8a90,ccb45000) at 
netbsd:nfs_unmount+0xb4
dounmount(ccb45000,0,ccaea800,ccb45000,0,0,cac74000,0,ccaeea58,bb9092c0) at 
netbsd:dounmount+0x110
sys_unmount(ccaea800,ccb86d00,ccb86d28,bbaedf0d,bbaed000,cac74000,1,bb9092c0,0,8
059330) at netbsd:sys_unmount+0x10e
syscall(ccb86d48,482f0023,23,bfbf0023,bbbb0023,0,bb9092c0,bfbfeae8,bbbe8110,0) 
at netbsd:syscall+0xaa

cheers
mark