af_netlink.c: make netlink_capable userns-aware

netlink_capable should check for permissions against the user
namespace owning the socket in question.

Changelog:
  Per Eric Dumazet advice, use sock_net(sk) instead of #ifdef.

Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
1 file changed