selftests/bpf: add testcases for fsession cookie

selftests/bpf: add testcases for fsession cookie

Test session cookie for fsession. Multiple fsession BPF progs is attached
to bpf_fentry_test1() and session cookie is read and write in the
testcase.

bpf_get_func_ip() will influence the layout of the session cookies, so we
test the cookie in two case: with and without bpf_get_func_ip().

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Link: https://lore.kernel.org/r/20260124062008.8657-13-dongml2@chinatelecom.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>