gfs2: Add gfs2_blk2rgrpd comment and fix incorrect use

Document when to use gfs2_blk2rgrpd for "inexact" resource group
matching.  Based on that, fix an incorrect use of gfs2_blk2rgrpd in
sweep_bh_for_rgrps.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
index d5f0d96..8b993e4 100644
--- a/fs/gfs2/bmap.c
+++ b/fs/gfs2/bmap.c
@@ -1133,7 +1133,7 @@ static int sweep_bh_for_rgrps(struct gfs2_inode *ip, struct gfs2_holder *rd_gh,
 			gfs2_assert_withdraw(sdp,
 				     gfs2_glock_is_locked_by_me(rd_gh->gh_gl));
 		} else {
-			rgd = gfs2_blk2rgrpd(sdp, bn, false);
+			rgd = gfs2_blk2rgrpd(sdp, bn, true);
 			ret = gfs2_glock_nq_init(rgd->rd_gl, LM_ST_EXCLUSIVE,
 						 0, rd_gh);
 			if (ret)