tcp: drain tcp_hostcache_list in between per-bucket locks

Explicitly drain the sbuf after completing each hash bucket
to minimize the work performed while holding the hash
bucket lock.

PR: 254333
MFC after: 2 weeks
Reviewed By: tuexen, jhb, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29483

(cherry picked from commit 869880463cc2ce64e2e6599eaec880a981f3ced6)