Fix bogus packet length calculation in the RX/LRO path, which can lead to

Fix bogus packet length calculation in the RX/LRO path, which can lead to
TSO tagging forwarded packets which shouldn't.  This will also fix the em(4)
watchdog timeouts seen after the em(4) TSO commit.

Thanks to Hrvoje Popovski for providing the infrastructure the reproduce
the issue, and test the fix.

Discussed with bluhm@.  OK claudio@