bpf: btf: Remove unused bits from uapi/linux/btf.h

bpf: btf: Remove unused bits from uapi/linux/btf.h

This patch does the followings:
1. Limit BTF_MAX_TYPES and BTF_MAX_NAME_OFFSET to 64k.  We can
   raise it later.

2. Remove the BTF_TYPE_PARENT and BTF_STR_TBL_ELF_ID.  They are
   currently encoded at the highest bit of a u32.
   It is because the current use case does not require supporting
   parent type (i.e type_id referring to a type in another BTF file).
   It also does not support referring to a string in ELF.

   The BTF_TYPE_PARENT and BTF_STR_TBL_ELF_ID checks are replaced
   by BTF_TYPE_ID_CHECK and BTF_STR_OFFSET_CHECK which are
   defined in btf.c instead of uapi/linux/btf.h.

3. Limit the BTF_INFO_KIND from 5 bits to 4 bits which is enough.
   There is unused bits headroom if we ever needed it later.

4. The root bit in BTF_INFO is also removed because it is not
   used in the current use case.

5. Remove BTF_INT_VARARGS since func type is not supported now.
   The BTF_INT_ENCODING is limited to 4 bits instead of 8 bits.

The above can be added back later because the verifier
ensures the unused bits are zeros.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>