ipv6: set msg_control_is_user in do_ipv6_getsockopt

While do_ipv6_getsockopt does not call the high-level recvmsg helper,
the msghdr eventually ends up being passed to put_cmsg anyway, and thus
needs msg_control_is_user set to the proper value.

Fixes: 1f466e1f ("net: cleanly handle kernel vs user buffers for ->msg_control")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>