socket: Handle the possibility of a protocol with no ctloutput

Add a default ctloutput handler and remove various NULL checks. This
fixes a problem wherein the generic SO_SETFIB handler did not check
whether the protocol has a ctloutput implementation before calling the
function pointer.

Reported by: syzkaller
Reviewed by: glebius
Fixes: caccbaef8e26 ("socket: Move SO_SETFIB handling to protocol layers")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49436