Refactor ssh_packet_hmac_verify to allow for direct buffer
This will make it easier to do Encrypt-then-MAC checks as those will be
on the direct encrypted data received before decrypting which means they
are not allocated in an ssh buffer at that point yet.
Signed-off-by: Dirkjan Bussink <firstname.lastname@example.org>
Reviewed-by: Jon Simons <email@example.com>
Reviewed-by: Jakub Jelen <firstname.lastname@example.org>