selftests/bpf: Some basic __uptr tests

Make sure the memory of uptrs have been mapped to the kernel properly.
Also ensure the values of uptrs in the kernel haven't been copied
to userspace.

It also has the syscall update_elem/delete_elem test to test the
pin/unpin code paths.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20241023234759.860539-9-martin.lau@linux.dev


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