selftests/bpf: Test kptr arrays and kptrs in nested struct fields.

selftests/bpf: Test kptr arrays and kptrs in nested struct fields.

Make sure that BPF programs can declare global kptr arrays and kptr fields
in struct types that is the type of a global variable or the type of a
nested descendant field in a global variable.

An array with only one element is special case, that it treats the element
like a non-array kptr field. Nested arrays are also tested to ensure they
are handled properly.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240523174202.461236-8-thinker.li@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>