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
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46378