selftests/bpf: factor out get_func_* tests for fsession

selftests/bpf: factor out get_func_* tests for fsession

The fsession is already supported by x86_64, arm64, riscv and s390, so we
don't need to disable it in the compile time according to the
architecture. Factor out the testings for it. Therefore, the testing can
be disabled for the architecture that doesn't support it manually.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20260224092208.1395085-4-dongml2@chinatelecom.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>