tcp: reduce memory footprint when listing tcp hostcache

In tcp_hostcache_list, the sbuf used would need a large (~2MB)
blocking allocation of memory (M_WAITOK), when listing a
full hostcache. This may stall the requestor for an indeterminate
time.

A further optimization is to return the expected userspace
buffersize right away, rather than preparing the output of
each current entry of the hostcase, provided by: @tuexen.

This makes use of the ready-made functions of sbuf to work
with sysctl, and repeatedly drain the much smaller buffer.

PR: 254333
MFC after: 2 weeks
Reviewed By: transport, tuexen
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29471

(cherry picked from commit cb0dd7e122b8936ad61a141e65ef8ef874bfebe5)