ipvlan: decouple l3s mode dependencies from other modes

Right now ipvlan has a hard dependency on CONFIG_NETFILTER and
otherwise it cannot be built. However, the only ipvlan operation
mode that actually depends on netfilter is l3s, everything else
is independent of it. Break this hard dependency such that users
are able to use ipvlan l3 mode on systems where netfilter is not
compiled in.

Therefore, this adds a hidden CONFIG_IPVLAN_L3S bool which is
defaulting to y when CONFIG_NETFILTER is set in order to retain
existing behavior for l3s. All l3s related code is refactored
into ipvlan_l3s.c that is compiled in when enabled.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Mahesh Bandewar <maheshb@google.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Martynas Pumputis <m@lambda.lt>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>