gfs2: Clean up out-of-bounds check in gfs2_rbm_from_block

We already have a function that checks if a block is within a resource
group, so use that in gfs2_rbm_from_block as well.

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