ipvlan: use pernet operations and restrict l3s hooks to master netns

commit 4fbae7d8 ("ipvlan: Introduce l3s mode") added
registration of netfilter hooks via nf_register_hooks().

This API provides the illusion of 'global' netfilter hooks by placing the
hooks in all current and future network namespaces.

In case of ipvlan the hook appears to be only needed in the namespace
that contains the ipvlan master device (i.e., usually init_net), so
placing them in all namespaces is not needed.

This switches ipvlan driver to pernet operations, and then only registers
hooks in namespaces where a ipvlan master device is set to l3s mode.

Extra care has to be taken when the master device is moved to another
namespace, as we might have to 'move' the netfilter hooks too.

This is done by storing the namespace the ipvlan port was created in.
On REGISTER event, do (un)register operations in the old/new namespaces.

This will also allow removal of the nf_register_hooks() in a future patch.

Cc: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>