btrfs: hold a ref on the root in btrfs_recover_log_trees

We replay the log into arbitrary fs roots, hold a ref on the root while
we're doing this.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-log.c

@@ -6163,6 +6163,10 @@ again:
 		tmp_key.offset = (u64)-1;
 
 		wc.replay_dest = btrfs_get_fs_root(fs_info, &tmp_key, true);
+		if (!IS_ERR(wc.replay_dest)) {
+			if (!btrfs_grab_fs_root(wc.replay_dest))
+				wc.replay_dest = ERR_PTR(-ENOENT);
+		}
 		if (IS_ERR(wc.replay_dest)) {
 			ret = PTR_ERR(wc.replay_dest);
 
@@ -6219,6 +6223,7 @@ again:
 		}
 
 		wc.replay_dest->log_root = NULL;
+		btrfs_put_fs_root(wc.replay_dest);
 		free_extent_buffer(log->node);
 		free_extent_buffer(log->commit_root);
 		kfree(log);