netfilter: ebtables: enforce CAP_NET_ADMIN

normal users are currently allowed to set/modify ebtables rules.
Restrict it to processes with CAP_NET_ADMIN.

Note that this cannot be reproduced with unmodified ebtables binary
because it uses SOCK_RAW.

Signed-off-by: Florian Westphal <fwestphal@astaro.com>
Cc: stable@kernel.org
Signed-off-by: Patrick McHardy <kaber@trash.net>