neta: improve TCP LRO

Use the appropriate function to flush correctly all entries. The old
code does not remove the element from the hash table, only from the
active queue.

Reviewed by: Peter Lei, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46433

(cherry picked from commit 5203dcce2527fd235648e2b855a13f62247eb2de)