FT/RRB: Pad RRB messages to at least minimum Ethernet frame length

FT/RRB: Pad RRB messages to at least minimum Ethernet frame length

Ethernet frames have minimum length of 64 octets and shorter frames may
end up getting arbitrary padding in the end. This would result in the
FT/RRB receiver rejecting the frame as an incorrectly protected one.
Work around this by padding the message so that it is never shorter than
the minimum Ethernet frame.

Unfortunately, this padding is apparently not enough with all Ethernet
devices and it is still possible to see extra two octet padding at the
end of the message even if larger frames are used (e.g., showed up with
128 byte frames). For now, work around this by trying to do AES-SIV
decryption with two octets shorter frame (ignore last two octets) if the
first attempt fails.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>