Merge branch 'fix-BTF-enum'

Yoshiki Komachi says:

====================
btf_enum_check_member() checked if the size of "enum" as a struct
member exceeded struct_size or not. Then, the function compared it
with the size of "int". Although the size of "enum" is 4-byte by
default (i.e., equivalent to "int"), the packing feature enables
us to reduce it, as illustrated by the following example:

struct A {
        char m;
        enum { E0, E1 } __attribute__((packed)) n;
};

With such a setup above, the bpf loader gave an error attempting
to load it:

------------------------------------------------------------------
...

[3] ENUM (anon) size=1 vlen=2
        E0 val=0
        E1 val=1
[4] STRUCT A size=2 vlen=2
        m type_id=2 bits_offset=0
        n type_id=3 bits_offset=8

[4] STRUCT A size=2 vlen=2
        n type_id=3 bits_offset=8 Member exceeds struct_size

libbpf: Error loading .BTF into kernel: -22.

------------------------------------------------------------------

The related issue was previously fixed by the commit 9eea98497951 ("bpf:
fix BTF verification of enums"). On the other hand, this series fixes
this issue as well, and adds a selftest program for it.

Changes in v2:
- change an example in commit message based on Andrii's review
- add a selftest program for packed "enum" type members in struct/union
====================

Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>