al_eth: 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/D46434

(cherry picked from commit 0b45d36510d8c629fcc49805bc64e5893f4ba63c)