net: netfilter: Deduplicate code in bpf_{xdp,skb}_ct_lookup

Move common checks inside the common function, and maintain the only
difference the two being how to obtain the struct net * from ctx.
No functional change intended.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220721134245.2450-7-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>