dm crypt: Initialize crypto wait structures

Use the DECLARE_CRYPTO_WAIT() macro to properly initialize the crypto
wait structures declared on stack before their use with
crypto_wait_req().

Fixes: 39d13a1a ("dm crypt: reuse eboiv skcipher for IV generation")
Fixes: bbb16584

 ("dm crypt: Implement Elephant diffuser for Bitlocker compatibility")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>