gfs2: don't allow releasepage to free bd still used for revokes

Before this patch, function gfs2_releasepage would free any bd
elements that had been used for the page being released. However,
those bd elements may still be queued to the sd_log_revokes list,
in which case we cannot free them until the revoke has been issued.

This patch adds additional checks for bds that are still being
used for revokes.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>

fs/gfs2/aops.c

@@ -805,11 +805,16 @@ int gfs2_releasepage(struct page *page, gfp_t gfp_mask)
 		bd = bh->b_private;
 		if (bd) {
 			gfs2_assert_warn(sdp, bd->bd_bh == bh);
-			if (!list_empty(&bd->bd_list))
-				list_del_init(&bd->bd_list);
 			bd->bd_bh = NULL;
 			bh->b_private = NULL;
-			kmem_cache_free(gfs2_bufdata_cachep, bd);
+			/*
+			 * The bd may still be queued as a revoke, in which
+			 * case we must not dequeue nor free it.
+			 */
+			if (!bd->bd_blkno && !list_empty(&bd->bd_list))
+				list_del_init(&bd->bd_list);
+			if (list_empty(&bd->bd_list))
+				kmem_cache_free(gfs2_bufdata_cachep, bd);
 		}
 
 		bh = bh->b_this_page;