VSOCK: Split vm_sockets.h into kernel/uapi

Split the vSockets header into kernel and UAPI parts.  The former gets the bits
that used to be in __KERNEL__ guards, while the latter gets everything that is
user-visible.  Tested by compiling vsock (+transport) and a simple user-mode
vSockets application.

Reported-by: David Howells <dhowells@redhat.com>
Acked-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Andy King <acking@vmware.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>