libbpf: Fix btf_dump's packed struct determination

Fix bug in btf_dump's logic of determining if a given struct type is
packed or not. The notion of "natural alignment" is not needed and is
even harmful in this case, so drop it altogether. The biggest difference
in btf_is_struct_packed() compared to its original implementation is
that we don't really use btf__align_of() to determine overall alignment
of a struct type (because it could be 1 for both packed and non-packed
struct, depending on specifci field definitions), and just use field's
actual alignment to calculate whether any field is requiring packing or
struct's size overall necessitates packing.

Add two simple test cases that demonstrate the difference this change
would make.

Fixes: ea2ce1ba99aa ("libbpf: Fix BTF-to-C converter's padding logic")
Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20221215183605.4149488-1-andrii@kernel.org

selftests/bpf: Add few corner cases to test padding handling of btf_dump

Add few hand-crafted cases and few randomized cases found using script
from [0] that tests btf_dump's padding logic.

[0] https://lore.kernel.org/bpf/85f83c333f5355c8ac026f835b18d15060725fcb.camel@ericsson.com/

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221212211505.558851-7-andrii@kernel.org

selftests/bpf: Fix btf_dump test under new clang

New clang version changed ([0]) type name in dwarf from "long int" to "long",
this is causing btf_dump tests to fail.

[0] https://github.com/llvm/llvm-project/commit/f6a561c4d6754b13165a49990e8365d819f64c86

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211008173139.1457407-1-fallentree@fb.com

selftests/bpf: add btf_dump BTF-to-C conversion tests

Add new test_btf_dump set of tests, validating BTF-to-C conversion
correctness. Tests rely on clang to generate BTF from provided C test
cases.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>