GFS2: Check for glock already held in gfs2_getxattr

Since the introduction of atomic_open, gfs2_getxattr can be
called with the glock already held, so we need to allow for
this.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Reported-by: David Teigland <teigland@redhat.com>
Tested-by: David Teigland <teigland@redhat.com>