btrfs: handle record_root_in_trans failure in create_pending_snapshot

record_root_in_trans can currently fail, so handle this failure
properly.

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