LRO: Don't merge ACK and non-ACK packets together

LRO was willing to merge ACK and non-ACK packets together. This
can cause incorrect th_ack values to be reported up the stack.
While non-ACKs are quite unlikely to appear in practice, LRO's
behaviour is against the spec. Make LRO unwilling to merge
packets with different TH_ACK flag values in order to fix the
issue.

Found by: Sysunit test
Differential Revision: https://reviews.freebsd.org/D33775
Reviewed by: rrs