Looking at the code, it is pretty obvious that sys/nfsserver/nfs_serv.c would
crash in readdirplus if VFS_VGET() returns an error other than EOPNOTSUPP. See
the "patch" file.
Fix: See patch file. NB, I haven't actually tested this.
Patch attached with submission follows:
How-To-Repeat: Probably doesn't occur in practice, since VFS_VGET() would not normally
return such an error.