xfs: work around bogus gcc warning in xfs_allocbt_init_cursor

GCC 4.6 complains about an array subscript is above array bounds when
using the btree index to index into the agf_levels array.  The only
two indices passed in are 0 and 1, and we have an assert insuring that.

Replace the trick of using the array index directly with using constants
in the already existing branch for assigning the XFS_BTREE_LASTREC_UPDATE
flag.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/xfs_alloc_btree.c

@@ -427,13 +427,16 @@ xfs_allocbt_init_cursor(
 
 	cur->bc_tp = tp;
 	cur->bc_mp = mp;
-	cur->bc_nlevels = be32_to_cpu(agf->agf_levels[btnum]);
 	cur->bc_btnum = btnum;
 	cur->bc_blocklog = mp->m_sb.sb_blocklog;
-
 	cur->bc_ops = &xfs_allocbt_ops;
-	if (btnum == XFS_BTNUM_CNT)
+
+	if (btnum == XFS_BTNUM_CNT) {
+		cur->bc_nlevels = be32_to_cpu(agf->agf_levels[XFS_BTNUM_CNT]);
 		cur->bc_flags = XFS_BTREE_LASTREC_UPDATE;
+	} else {
+		cur->bc_nlevels = be32_to_cpu(agf->agf_levels[XFS_BTNUM_BNO]);
+	}
 
 	cur->bc_private.a.agbp = agbp;
 	cur->bc_private.a.agno = agno;