ANDROID: GKI: add vendor padding variable in struct nf_conn

Some vendors want to add things to 'struct nf_conn', so give them a u64
where they can then have a pointer off to their private data and they
can do whatever they want to do without breaking or changing any abi for
anyone else.

Note, usually an android trace hook is also needed to use this properly,
so be aware that this will be required as well.

Bug: 171013716
Change-Id: I245c162ee3fb083e3f39cf7bec3bd78cb624e005
Signed-off-by: Aditya Agrawal <aditya.a1@samsung.com>