netfilter: nf_tables: add static key to skip retpoline workarounds

If CONFIG_RETPOLINE is enabled nf_tables avoids indirect calls for
builtin expressions.

On newer cpus indirect calls do not go through the retpoline thunk
anymore, even for RETPOLINE=y builds.

Just like with the new tc retpoline wrappers:
Add a static key to skip the if / else if cascade if the cpu
does not require retpolines.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>