tcp: LRO code to deal with all 12 TCP header flags

TCP per RFC793 has 4 reserved flag bits for future use. One
of those bits may be used for Accurate ECN.
This patch is to include these bits in the LRO code to ease
the extensibility if/when these bits are used.

Reviewed By: hselasky, rrs, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34127