net: cleanly handle kernel vs user buffers for ->msg_control

The msg_control field in struct msghdr can either contain a user
pointer when used with the recvmsg system call, or a kernel pointer
when used with sendmsg.  To complicate things further kernel_recvmsg
can stuff a kernel pointer in and then use set_fs to make the uaccess
helpers accept it.

Replace it with a union of a kernel pointer msg_control field, and
a user pointer msg_control_user one, and allow kernel_recvmsg operate
on a proper kernel pointer using a bitfield to override the normal
choice of a user pointer for recvmsg.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>