bpf: Embed kernel CONFIG check into the if statement in bpf_getsockopt

bpf: Embed kernel CONFIG check into the if statement in bpf_getsockopt

This patch moves the "#ifdef CONFIG_XXX" check into the "if/else"
statement itself.  The change is done for the bpf_getsockopt()
function only.  It will make the latter patches easier to follow
without the surrounding ifdef macro.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20220902002906.2893572-1-kafai@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>