tcp: Make hostcache.cache_count MPSAFE by using a counter_u64_t

Addressing the underlying root cause for cache_count to
show unexpectedly high values, by protecting all arithmetic on
that global variable by using counter(9).

PR: 254333
Reviewed By: tuexen, transport
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29510