btrfs: return original error code when failing from option parsing

It's not good to overwrite -ENOMEM using -EINVAL when failing from mount
option parsing, so just return original error code.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>