bpf: Constify struct btf_kind_operations

struct btf_kind_operations are not modified in BTF.

Constifying this structures moves some data to a read-only section,
so increase overall security, especially when the structure holds
some function pointers.

On a x86_64, with allmodconfig:

Before:
======
text data bss dec hex filename
184320 7091 548 191959 2edd7 kernel/bpf/btf.o

After:
=====
text data bss dec hex filename
184896 6515 548 191959 2edd7 kernel/bpf/btf.o

Signed-off-by: Christophe JAILLET <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/r/9192ab72b2[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/btf.c

···
 	btf_verifier_log(env, "type_id=%u", t->type);
 }
 
-static struct btf_kind_operations modifier_ops = {
+static const struct btf_kind_operations modifier_ops = {
 	.check_meta = btf_ref_type_check_meta,
 	.resolve = btf_modifier_resolve,
 	.check_member = btf_modifier_check_member,
···
 	.show = btf_modifier_show,
 };
 
-static struct btf_kind_operations ptr_ops = {
+static const struct btf_kind_operations ptr_ops = {
 	.check_meta = btf_ref_type_check_meta,
 	.resolve = btf_ptr_resolve,
 	.check_member = btf_ptr_check_member,
···
 	btf_verifier_log(env, "%s", btf_type_kflag(t) ? "union" : "struct");
 }
 
-static struct btf_kind_operations fwd_ops = {
+static const struct btf_kind_operations fwd_ops = {
 	.check_meta = btf_fwd_check_meta,
 	.resolve = btf_df_resolve,
 	.check_member = btf_df_check_member,
···
 	__btf_array_show(btf, t, type_id, data, bits_offset, show);
 }
 
-static struct btf_kind_operations array_ops = {
+static const struct btf_kind_operations array_ops = {
 	.check_meta = btf_array_check_meta,
 	.resolve = btf_array_resolve,
 	.check_member = btf_array_check_member,
···
 	__btf_struct_show(btf, t, type_id, data, bits_offset, show);
 }
 
-static struct btf_kind_operations struct_ops = {
+static const struct btf_kind_operations struct_ops = {
 	.check_meta = btf_struct_check_meta,
 	.resolve = btf_struct_resolve,
 	.check_member = btf_struct_check_member,
···
 	btf_show_end_type(show);
 }
 
-static struct btf_kind_operations enum_ops = {
+static const struct btf_kind_operations enum_ops = {
 	.check_meta = btf_enum_check_meta,
 	.resolve = btf_df_resolve,
 	.check_member = btf_enum_check_member,
···
 	btf_show_end_type(show);
 }
 
-static struct btf_kind_operations enum64_ops = {
+static const struct btf_kind_operations enum64_ops = {
 	.check_meta = btf_enum64_check_meta,
 	.resolve = btf_df_resolve,
 	.check_member = btf_enum_check_member,
···
 	btf_verifier_log(env, ")");
 }
 
-static struct btf_kind_operations func_proto_ops = {
+static const struct btf_kind_operations func_proto_ops = {
 	.check_meta = btf_func_proto_check_meta,
 	.resolve = btf_df_resolve,
 	/*
···
 	return 0;
 }
 
-static struct btf_kind_operations func_ops = {
+static const struct btf_kind_operations func_ops = {
 	.check_meta = btf_func_check_meta,
 	.resolve = btf_func_resolve,
 	.check_member = btf_df_check_member,