crypto/krb5, rxrpc: Fix lack of pre-decrypt/pre-verify length checks

crypto/krb5, rxrpc: Fix lack of pre-decrypt/pre-verify length checks

Change the krb5 crypto library to provide facilities to precheck the length
of the message about to be decrypted or verified.

Fix AF_RXRPC to make use of this to validate DATA packets secured with
RxGK.

Fixes: 9d1d2b59341f ("rxrpc: rxgk: Implement the yfs-rxgk security class (GSSAPI)")
Closes: https://sashiko.dev/#/patchset/20260511160753.607296-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: Simon Horman <horms@kernel.org>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: linux-afs@lists.infradead.org
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>
Link: https://patch.msgid.link/20260515230516.2718212-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>