bpf: selftests: Add sk_state to bpf_tcp_helpers.h

Add sk_state define to bpf_tcp_helpers.h.  Rename the existing
global variable "sk_state" in the kfunc_call test to "sk_state_res".

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210824173013.3977316-1-kafai@fb.com