selftests/bpf: Add tests for KF_RCU_PROTECTED

Add a couple of test cases to ensure RCU protection is kicked in
automatically, and the return type is as expected.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250917032755.4068726-3-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>