tcp: initialize the LRO hash table with correct size

There will at most lro_entries entries in the LRO hash table. So no
need to take lro_mbufs into account, which only results in the
LRO hash table being too large and therefore wasting memory.

Reviewed by: rrs
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46378

(cherry picked from commit aa6c490bf80fcef15cfc0d3f562fae19ef2375aa)