An intermittent problem with NFSv4 exporting of ZFS snapshots was

An intermittent problem with NFSv4 exporting of ZFS snapshots was
reported to the freebsd-fs mailing list. I believe the problem was
caused by the Readdir operation using VFS_VGET() for a snapshot file entry
instead of VOP_LOOKUP(). This would not occur for NFSv3, since it
will do a VFS_VGET() of "." which fails with ENOTSUPP at the beginning
of the directory, whereas NFSv4 does not check "." or "..". This
patch adds a call to VFS_VGET() for the directory being read to check
for ENOTSUPP.
I also observed that the mount_on_fileid and fsid attributes were
not correct at the snapshot's auto mountpoints when looking at packet
traces for the Readdir. This patch fixes the attributes by doing a check
for different v_mount structure, even if the vnode v_mountedhere is not
set.

Reported by:	jas@cse.yorku.ca
Tested by:	jas@cse.yorku.ca
Reviewed by:	asomers
MFC after:	1 week