selftests/bpf: Add test for narrow ctx load for pointer args

Ensure that performing narrow ctx loads other than size == 8 are
rejected when the argument is a pointer type.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241212092050.3204165-3-memxor@gmail.com


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