GFS2: lops.c: replace 0 by NULL for pointers

Sparse warning: fs/gfs2/lops.c:78:29:
"warning: Using plain integer as NULL pointer"

Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/lops.c

@@ -75,7 +75,7 @@ static void maybe_release_space(struct gfs2_bufdata *bd)
 	unsigned int index = bd->bd_bh->b_blocknr - gl->gl_name.ln_number;
 	struct gfs2_bitmap *bi = rgd->rd_bits + index;
 
-	if (bi->bi_clone == 0)
+	if (bi->bi_clone == NULL)
 		return;
 	if (sdp->sd_args.ar_discard)
 		gfs2_rgrp_send_discards(sdp, rgd->rd_data0, bd->bd_bh, bi, 1, NULL);