dm-crypt: Allow to specify the integrity key size as option

dm-crypt: Allow to specify the integrity key size as option

For the MAC based integrity operation, the integrity key size (i.e.
key_mac_size) is currently set to the digest size of the used digest.

For wrapped key HMAC algorithms, the key size is independent of the
cryptographic key size. So there is no known size of the mac key in
such cases. The desired key size can optionally be specified as argument
when the dm-crypt device is configured via 'integrity_key_size:%u'.
If no integrity_key_size argument is specified, the mac key size
is still set to the digest size, as before.

Increase version number to 1.28.0 so that support for the new
argument can be detected by user space (i.e. cryptsetup).

Signed-off-by: Ingo Franzki <ifranzki@linux.ibm.com>
Reviewed-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>