[NETFILTER] nfnetlink: unconditionally require CAP_NET_ADMIN

This patch unconditionally requires CAP_NET_ADMIN for all nfnetlink
messages.  It also removes the per-message cap_required field, since all
existing subsystems use CAP_NET_ADMIN for all their messages anyway.

Patrick McHardy owes me a beer if we ever need to re-introduce this.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>