In sendit(), if mp->msg_control is present, then in sockargs() we are allocating
mbuf to store mp->msg_control. Later in kern_sendit(), call to getsock_cap(),
will check validity of file pointer passed, if this fails EBADF is returned but
mbuf allocated in sockargs() is not freed. Fix this possible leak.

Submitted by: Lohith Bellad <lohith.bellad@me.com>
Reviewed by: adrian
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D7910