dm-crypt: switch to using the crc32 library

Now that the crc32() library function takes advantage of
architecture-specific optimizations, it is unnecessary to go through the
crypto API.  Just use crc32().  This is much simpler, and it improves
performance due to eliminating the crypto API overhead.  (However, this
only affects the TCW IV mode of dm-crypt, which is a compatibility mode
that is rarely used compared to other dm-crypt modes.)

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>