md/raid5: remove redundant check in stripe_add_to_batch_list()

The stripe_add_to_batch_list() function is called only if
stripe_can_batch() returned true, so there is no need for double check.

Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
Cc: Neil Brown <neilb@suse.com>
Cc: linux-raid@vger.kernel.org
Signed-off-by: NeilBrown <neilb@suse.com>