cryptodev: Allow some CIOCCRYPT operations with an empty payload.

If an operation would generate a MAC output (e.g. for digest operation
or for an AEAD or EtA operation), then an empty payload buffer is
valid. Only reject requests with an empty buffer for "plain" cipher
sessions.

Some of the AES-CCM NIST KAT vectors use an empty payload.

While here, don't advance crp_payload_start for requests that use an
empty payload with an inline IV. (*)

Reported by: syzbot+d4b94fbd9a44b032f428@syzkaller.appspotmail.com (*)
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32109