swrap: don't read the callers msg_control buffer in swrap_recvmsg_before_unix()

For recvmsg() msg_control is a write only buffer, that is filled by the kernel, but the kernel won't read from that buffer. So we shouldn't read from (copy) it either.

Signed-off-by: Stefan Metzmacher metze@samba.org Reviewed-by: Andreas Schneider asn@samba.org