ANDROID: fix KMI after enabling CONFIG_IPVLAN

Enabling CONFIG_IPVLAN causes CONFIG_NET_L3_MASTER_DEV to
be selected which adds fields to `struct net_device` and
`struct netns_ipv4`.

For net_device, an ANDROID_KABI_BACKPORT_OK() field is used.
For netns_ipv4, the new u8 fields are placed in an alignment
padding area so they do not change the struct size or any
offsets to other fields.

Bug: 391829285
Change-Id: I2ec2f32a179aa6c5728ff7168118b4cc72ce7ea3
Signed-off-by: Todd Kjos <tkjos@google.com>