It should be checking flags, not msg->msg_flags.  It's ->sendmsg()
instances that need to look for that in ->msg_flags, ->recvmsg() ones
(including the other ->recvmsg() instance in that file, as well as
unix_dgram_recvmsg() this one claims to be imitating) check in flags.
Braino had been introduced in commit dcda13 ("caif: Bugfix - use MSG_TRUNC
in receive") back in 2010, so it goes quite a while back.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>