netlink: kill netlink_set_nonroot

Replace netlink_set_nonroot by one new field `flags' in
struct netlink_kernel_cfg that is passed to netlink_kernel_create.

This patch also renames NL_NONROOT_* to NL_CFG_F_NONROOT_* since
now the flags field in nl_table is generic (so we can add more
flags if needed in the future).

Also adjust all callers in the net-next tree to use these flags
instead of netlink_set_nonroot.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>