net: Move "struct net" declaration inside the __KERNEL__ macro guard

This patch reduces namespace pollution by moving the "struct net" declaration
out of the userspace-facing portion of linux/netlink.h.  It has no impact on
the kernel.

(This came up because we have several C++ applications which use "net" as a
namespace name.)

Signed-off-by: Ollie Wild <aaw@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>