MFC r283291: don't use CALLOUT_MPSAFE with callout_init()

The main purpose of this MFC is to reduce conflicts for other merges.
Parts of the original change have already "trickled down" via individual MFCs.

MFC 277709:
Use an sbuf to generate the output of the net.inet.tcp.hostcache.list
sysctl to avoid a possible buffer overflow if the cache grows while the
text is being generated.

PR: 172675

MFC 277709:
Use an sbuf to generate the output of the net.inet.tcp.hostcache.list
sysctl to avoid a possible buffer overflow if the cache grows while the
text is being generated.

PR: 172675