GFS2: Stop block extents at the end of bitmaps

This patch stops multiple block allocations if a nonzero
return code is received from gfs2_rbm_from_block. Without
this patch, if enough pressure is put on the file system,
you get a kernel warning quickly followed by:
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffffa04f47e8>] gfs2_alloc_blocks+0x2c8/0x880 [gfs2]
With this patch, things run normally.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/rgrp.c

@@ -1834,8 +1834,7 @@ static void gfs2_alloc_extent(const struct gfs2_rbm *rbm, bool dinode,
 	block++;
 	while (*n < elen) {
 		ret = gfs2_rbm_from_block(&pos, block);
-		WARN_ON(ret);
-		if (gfs2_testbit(&pos) != GFS2_BLKST_FREE)
+		if (ret || gfs2_testbit(&pos) != GFS2_BLKST_FREE)
 			break;
 		gfs2_trans_add_bh(pos.rgd->rd_gl, pos.bi->bi_bh, 1);
 		gfs2_setbit(&pos, true, GFS2_BLKST_USED);