OPENAFS-SA-2017-001: rx: Sanity-check received MTU and twind values

OPENAFS-SA-2017-001: rx: Sanity-check received MTU and twind values

Rather than blindly trusting the values received in the
(unauthenticated) ack packet trailer, apply some minmial sanity checks
to received values.  natMTU and regular MTU values are subject to
Rx minmium/maximum packet sizes, and the transmit window cannot drop
below one without risk of deadlock.

The maxDgramPackets value that can also be present in the trailer
already has sufficient sanity checking.

Extremely low MTU values (less than 28 == RX_HEADER_SIZE) can cause us
to set a negative "maximum usable data" size that gets used as an
(unsigned) packet length for subsequent allocation and computation,
triggering an assertion when the connection is used to transmit data.

FIXES 134450

Change-Id: I37698ff166da47a57aa0d1962ae8effc74e30851