Improvements of BTF sanitizing for old kernels

[Timur Chernykh <tim.cherry.co@gmail.com>]

Hello everyone. Recently I've encountered some issues with loading on debian 10 with kernel version `4.19.0-[17-22]`.

This first one consists of when the kernel loads BTF with specified min-CORE and libbpf does some sanitizing on those, then it "translates" func_proto to enum. But if `func_proto` has no names for it's parameters then kernel verifier fails with "Invalid name" error. This error caused by enum members must has a valid C identifier, but there's might be no names generated.

The second issue this PR supposed to fix is missed check whether kernel supports the `kind flag` or not. Let's say we're compiling the BPF object on new kernel with a new compiler. There's 99% percent chance that compiler will use this `kind flag`. But of we're loading on old kernels that have no support of this, so the loading will fail in multiple places.

- The type metadata check. Old BTF_INFO_MASK prohibits 31d bit
- Integer bitfield verification. Because old kernel waits for an offset-only encoding for the btf_member->offset field, but it's encoded in different way in case when the `kind flag` is set.

This PR contains:
- Enum names generation during sanitizing process for `func_proto` kind, when it being translate to `enum` kind.
- The feature check whether kernel supports the kind flag or not
- Kind flag sanitizing if kernel doesn't support one
- Struct/enum bitfield members sanitizing by generation a proper replacement the type of bitfield with corresponding integer type with same bit size

From: Timur Chernykh <tim.cherry.co@gmail.com>
To: bpf@vger.kernel.org
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Subject: Improvements of BTF sanitizing for old kernels
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