bpf: Fix using struct btf_ptr on 32-bit

struct btf_ptr {
        void *ptr;
        __u32 type_id;
        __u32 flags;
};

test_task_btf:PASS:bpf_iter_task_btf__open_and_load 0 nsec
do_btf_read:PASS:attach_iter 0 nsec
do_btf_read:PASS:create_iter 0 nsec
do_btf_read:PASS:read 0 nsec
do_btf_read:FAIL:check for btf representation of task_struct in iter data unexpected check for btf representation of task_struct in iter data: '(struct task_struct)' is not a substring of 'Raw BTF task
'
test_task_btf:FAIL:no task iteration, did BPF program run? unexpected no task iteration, did BPF program run?: actual 0 == expected 0
torvalds#16/13   bpf_iter/task_btf:FAIL

static struct btf_ptr ptr = {};
long ret;
[...]
ret = bpf_seq_printf_btf(seq, &ptr, sizeof(ptr), 0);

--> -EINVAL

Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>

Author: guidosarducci

include/uapi/linux/bpf.h

@@ -7526,7 +7526,7 @@ struct bpf_sk_lookup {
  * Display flags - BTF_F_* - are passed to bpf_snprintf_btf separately.
  */
 struct btf_ptr {
-	void *ptr;
+	__bpf_md_ptr(void *, ptr);
 	__u32 type_id;
 	__u32 flags;		/* BTF ptr flags; unused at present. */
 };

tools/include/uapi/linux/bpf.h

@@ -7526,7 +7526,7 @@ struct bpf_sk_lookup {
  * Display flags - BTF_F_* - are passed to bpf_snprintf_btf separately.
  */
 struct btf_ptr {
-	void *ptr;
+	__bpf_md_ptr(void *, ptr);
 	__u32 type_id;
 	__u32 flags;		/* BTF ptr flags; unused at present. */
 };