linux(4): Fix MSG_CTRUNC handling in recvmsg()

The MSG_CTRUNC flag of the msg_flags member of the message header is
set uppon successful completition if the control data was truncated.
Upon return from a successful call msg_controllen should contain the
length of the control message sequence.

Fixes: 0eda2cea
MFC after: 1 week

(cherry picked from commit 2467ccddc0e0f5c719eddaece69a7aa0589b6449)