crc-t10dif: Pick better transform if one becomes available

T10 CRC library is linked into the kernel thanks to block and SCSI. The
crypto accelerators are typically loaded later as modules and are
therefore not available when the T10 CRC library is initialized.

Use the crypto notifier facility to trigger a switch to a better algorithm
if one becomes available after the initial hash has been registered. Use
RCU to protect the original transform while the new one is being set up.

Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>