Fix leak of memory and file refs with sendmsg(2) over unix domain sockets.

When sendmsg(2) sucessfully internalized one SCM_RIGHTS control
message, but failed to process some other control message later, both
file references and filedescent memory needs to be freed. This was not
done, only mbuf chain was freed.

Noted, test case written, reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D21000