GFS2: Ensure rindex is uptodate for fallocate

This patch fixes a problem whereby gfs2_grow was failing and causing GFS2
to assert. The problem was that when GFS2's fallocate operation tried to
acquire an "allocation" it made sure the rindex was up to date, and if not,
it called gfs2_rindex_update. However, if the file being fallocated was
the rindex itself, it was already locked at that point. By calling
gfs2_rindex_update at an earlier point in time, we bring rindex up to date
and thereby avoid trying to lock it when the "allocation" is acquired.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>