gfs2: Fix loop in gfs2_rbm_find

Fix the resource group wrap-around logic in gfs2_rbm_find that commit
e579ed4f broke.  The bug can lead to unnecessary repeated scanning of the
same bitmaps; there is a risk that future changes will turn this into an
endless loop.

Fixes: e579ed4f

 ("GFS2: Introduce rbm field bii")
Cc: stable@vger.kernel.org # v3.13+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>