[NET]: unify netlink kernel socket recognition

There are currently two ways to determine whether the netlink socket is a
kernel one or a user one. This patch creates a single inline call for
this purpose and unifies all the calls in the af_netlink.c

No similar calls are found outside af_netlink.c.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>