net: check kern before calling security subsystem

Before calling capable(CAP_NET_RAW) check if this operations is on behalf
of the kernel or on behalf of userspace.  Do not do the security check if
it is on behalf of the kernel.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>