dm crypt: fix error return code in crypt_ctr()

Fix to return error code -ENOMEM from the mempool_create_kmalloc_pool()
error handling case instead of 0, as done elsewhere in this function.

Fixes: ef43aa38 ("dm crypt: add cryptographic data integrity protection (authenticated encryption)")
Cc: stable@vger.kernel.org # 4.12+
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>