netfilter: nfnetlink: return ENOMEM if we fail to create netlink socket

With this patch, nfnetlink returns -ENOMEM instead of -EPERM if we
fail to create the nfnetlink netlink socket during the module
loading. This is exactly what rtnetlink does in this case.

Ideally, it would be better if we propagate the error that has
happened in netlink_kernel_create(), however, this function still
does not implement this yet.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>