btrfs: validate root::reloc_root after recording root in trans

If we fail to setup a root->reloc_root in a different thread that path
will error out, however it still leaves root->reloc_root NULL but would
still appear set up in the transaction.  Subsequent calls to
btrfs_record_root_in_transaction would succeed without attempting to
create the reloc root, as the transid has already been updated.

Handle this case by making sure we have a root->reloc_root set after a
btrfs_record_root_in_transaction call so we don't end up dereferencing a
NULL pointer.
Reported-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
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/relocation.c

@@ -2082,6 +2082,13 @@ struct btrfs_root *select_reloc_root(struct btrfs_trans_handle *trans,
 			return ERR_PTR(ret);
 		root = root->reloc_root;
 
+		/*
+		 * We could have raced with another thread which failed, so
+		 * root->reloc_root may not be set, return ENOENT in this case.
+		 */
+		if (!root)
+			return ERR_PTR(-ENOENT);
+
 		if (next->new_bytenr != root->node->start) {
 			/*
 			 * We just created the reloc root, so we shouldn't have
@@ -2579,6 +2586,14 @@ static int relocate_tree_block(struct btrfs_trans_handle *trans,
 			ret = btrfs_record_root_in_trans(trans, root);
 			if (ret)
 				goto out;
+			/*
+			 * Another thread could have failed, need to check if we
+			 * have reloc_root actually set.
+			 */
+			if (!root->reloc_root) {
+				ret = -ENOENT;
+				goto out;
+			}
 			root = root->reloc_root;
 			node->new_bytenr = root->node->start;
 			btrfs_put_root(node->root);