btrfs: send: remove unnecessary return variable from process_new_xattr()

There's no need for the 'ret' variable, we can just return directly the
result of the call to iterate_dir_item().

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>