Btrfs: check if items are ordered when a leaf is marked dirty

To ease finding bugs during development related to modifying btree leaves
in such a way that it makes its items not sorted by key anymore. Since this
is an expensive check, it's only enabled if CONFIG_BTRFS_FS_CHECK_INTEGRITY
is set, which isn't meant to be enabled for regular users.

Signed-off-by: Filipe David Borba Manana <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/disk-io.c

···
 		__percpu_counter_add(&root->fs_info->dirty_metadata_bytes,
 				     buf->len,
 				     root->fs_info->dirty_metadata_batch);
+#ifdef CONFIG_BTRFS_FS_CHECK_INTEGRITY
+	if (btrfs_header_level(buf) == 0 && check_leaf(root, buf)) {
+		btrfs_print_leaf(root, buf);
+		ASSERT(0);
+	}
+#endif
 }
 
 static void __btrfs_btree_balance_dirty(struct btrfs_root *root,