bpf: Remove recursion call in btf_struct_access

Andrii suggested we can simply jump to again label
instead of making recursion call.

Suggested-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200825192124.710397-7-jolsa@kernel.org

kernel/bpf/btf.c

@@ -3931,14 +3931,13 @@ int btf_struct_access(struct bpf_verifier_log *log,
 		/* Only allow structure for now, can be relaxed for
 		 * other types later.
 		 */
-		elem_type = btf_type_skip_modifiers(btf_vmlinux,
-						    array_elem->type, NULL);
-		if (!btf_type_is_struct(elem_type))
+		t = btf_type_skip_modifiers(btf_vmlinux, array_elem->type,
+					    NULL);
+		if (!btf_type_is_struct(t))
 			goto error;
 
-		off = (off - moff) % elem_type->size;
-		return btf_struct_access(log, elem_type, off, size, atype,
-					 next_btf_id);
+		off = (off - moff) % t->size;
+		goto again;
 
 error:
 		bpf_log(log, "access beyond struct %s at off %u size %u\n",