tcp: tcp: allow SOL_SOCKET-level socket options via sysctl interface

tcp: tcp: allow SOL_SOCKET-level socket options via sysctl interface

When using the sysctl interface for setting a SOL_SOCKET-level socket
option, the TCP handler refers to the IP handler, which only handles
SO_SETFIB and SO_MAX_PACING_RATE.
So call sosetopt(), which handles all SOL_SOCKET-level options.
Then you can use tcpsso with SOL_SOCKET-level socket options as
expected.

Reported by:		rscheff
Reviewed by:		glebius, rscheff
MFC after:		1 week
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D42985