MFC r340631:

Do proper copyin of control message data in the Linux sendmsg syscall.

Instead of calling m_append with a user address, allocate an mbuf cluster
and copy data into it using copyin. For the SCM_CREDS case, instead of
zeroing a stack variable and appending that to the mbuf, zero part of the
mbuf cluster directly. One mbuf cluster is also the size limit used by
the FreeBSD sendmsg syscall (uipc_syscalls.c:sockargs()).

PR: 217901
Reviewed by: kib