Btrfs: fix BUG_ON in btrfs_mark_buffer_dirty

This can only happen with CONFIG_BTRFS_FS_CHECK_INTEGRITY=y.

Commit 1ba98d0 ("Btrfs: detect corruption when non-root leaf has zero item")
assumes that a leaf is its root when leaf->bytenr == btrfs_root_bytenr(root),
however, we should not use btrfs_root_bytenr(root) since it's mainly got
updated during committing transaction.  So the check can fail when doing
COW on this leaf while it is a root.

This changes to use "if (leaf == btrfs_root_node(root))" instead, just like
how we check whether leaf is a root in __btrfs_cow_block().

Fixes: 1ba98d086fe3 (Btrfs: detect corruption when non-root leaf has zero item)
Cc: stable@vger.kernel.org  # 4.8+
Reported-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>

fs/btrfs/disk-io.c

@@ -572,13 +572,17 @@ static noinline int check_leaf(struct btrfs_root *root,
 		 * open_ctree() some roots has not yet been set up.
 		 */
 		if (!IS_ERR_OR_NULL(check_root)) {
+			struct extent_buffer *eb;
+
+			eb = btrfs_root_node(check_root);
 			/* if leaf is the root, then it's fine */
-			if (leaf->start !=
-			    btrfs_root_bytenr(&check_root->root_item)) {
+			if (leaf != eb) {
 				CORRUPT("non-root leaf's nritems is 0",
-					leaf, root, 0);
+					leaf, check_root, 0);
+				free_extent_buffer(eb);
 				return -EIO;
 			}
+			free_extent_buffer(eb);
 		}
 		return 0;
 	}