crypto: xts - drop xts_check_key()

xts_check_key() is obsoleted by xts_verify_key(). Over time XTS crypto
drivers adopted the newer xts_verify_key() variant, but xts_check_key()
is still used by a number of drivers. Switch drivers to use the newer
xts_verify_key() and make a couple of cleanups. This allows us to drop
xts_check_key() completely and avoid redundancy.

Signed-off-by: Vladis Dronov <vdronov@redhat.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Nicolai Stange <nstange@suse.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>