bpf: Make function btf_name_offset_valid static

Initially in commit 69b693f0ae ("bpf: btf: Introduce BPF Type Format
(BTF)") the function 'btf_name_offset_valid' was introduced as static
function it was later on changed to a non-static one, and then finally
in commit 23127b33ec ("bpf: Create a new btf_name_by_offset() for
non type name use case") the function prototype was removed.

Revert back to original implementation and make the function static.
Remove warning triggered with W=1:

  kernel/bpf/btf.c:470:6: warning: no previous prototype for 'btf_name_offset_valid' [-Wmissing-prototypes]

Fixes: 23127b33ec ("bpf: Create a new btf_name_by_offset() for non type name use case")
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/btf.c

@@ -467,7 +467,7 @@ static const struct btf_kind_operations *btf_type_ops(const struct btf_type *t)
 	return kind_ops[BTF_INFO_KIND(t->info)];
 }
 
-bool btf_name_offset_valid(const struct btf *btf, u32 offset)
+static bool btf_name_offset_valid(const struct btf *btf, u32 offset)
 {
 	return BTF_STR_OFFSET_VALID(offset) &&
 		offset < btf->hdr.str_len;