dm crypt: use crypto_shash_digest() in crypt_iv_tcw_whitening()

Simplify crypt_iv_tcw_whitening() by using crypto_shash_digest() instead
of an init+update+final sequence.  This should also improve performance.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>