[GFS2] nfsd readdirplus assertion failure

Glock assertion failure found in '07 NFS connectathon. One of the NFSDs
is doing a "readdirplus" procedure call. It passes the logic into
gfs2_readdir() where it obtains its directory inode glock. This is then
followed by filehandle construction that invokes lookup code. It hits
the assertion failure while trying to obtain the inode glock again
inside gfs2_drevalidate().

This patch bypasses the recursive glock call if caller already holds the
lock.

Signed-off-by: S. Wendy Cheng <wcheng@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>