bpf/btf: Fix BTF verification of enum members in struct/union

btf_enum_check_member() was currently sure to recognize the size of
"enum" type members in struct/union as the size of "int" even if
its size was packed.

This patch fixes BTF enum verification to use the correct size
of member in BPF programs.

Fixes: 179cde8c ("bpf: btf: Check members of struct/union")
Signed-off-by: Yoshiki Komachi <komachi.yoshiki@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/1583825550-18606-2-git-send-email-komachi.yoshiki@gmail.com