rx: Tidy up rxi_CheckCall()'s mtuout handling

rx: Tidy up rxi_CheckCall()'s mtuout handling

We don't actually do anything that matters if lastPacketSizeSeq
is set and lastPacketSize is zero, so zero both when we're cleaning
up.

lastPacketSize and lastPacketSizeSeq are set together in
rxi_SendPacket (and rxi_SendPacketList), when we are sending a packet
larger than the current estimate of the peer's maxPacketSize.

The two fields are checked together during ack processing, but
rxi_CheckCall() only checks lastPacketSize, ignoring lastPacketSizeSeq.

Change-Id: I4e52bed0900b5551859200699f114f5d5a61581c
Reviewed-on: http://gerrit.openafs.org/11633
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>