netfilter: nf_tables: avoid retpoline overhead for some ct expression calls

nft_ct expression cannot be made builtin to nf_tables without also
forcing the conntrack itself to be builtin.

However, this can be avoided by splitting retrieval of a few
selector keys that only need to access the nf_conn structure,
i.e. no function calls to nf_conntrack code.

Many rulesets start with something like
"ct status established,related accept"

With this change, this no longer requires an indirect call, which
gives about 1.8% more throughput with a simple conntrack-enabled
forwarding test (retpoline thunk used).

Signed-off-by: Florian Westphal <fw@strlen.de>