bpf: Create a new btf_name_by_offset() for non type name use case

The current btf_name_by_offset() is returning "(anon)" type name for
the offset == 0 case and "(invalid-name-offset)" for the out-of-bound
offset case.

It fits well for the internal BTF verbose log purpose which
is focusing on type.  For example,
offset == 0 => "(anon)" => anonymous type/name.
Returning non-NULL for the bad offset case is needed
during the BTF verification process because the BTF verifier may
complain about another field first before discovering the name_off
is invalid.

However, it may not be ideal for the newer use case which does not
necessary mean type name.  For example, when logging line_info
in the BPF verifier in the next patch, it is better to log an
empty src line instead of logging "(anon)".

The existing bpf_name_by_offset() is renamed to __bpf_name_by_offset()
and static to btf.c.

A new bpf_name_by_offset() is added for generic context usage.  It
returns "\0" for name_off == 0 (note that btf->strings[0] is "\0")
and NULL for invalid offset.  It allows the caller to decide
what is the best output in its context.

The new btf_name_by_offset() is overlapped with btf_name_offset_valid().
Hence, btf_name_offset_valid() is removed from btf.h to keep the btf.h API
minimal.  The existing btf_name_offset_valid() usage in btf.c could also be
replaced later.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>