bpf: fix argument type in bpf_loop documentation

The `index` argument to bpf_loop() is threaded as an u64.
This lead in a subtle verifier denial where clang cloned the argument
in another register[1].

[1] https://github.com/systemd/systemd/pull/34650#issuecomment-2401092895

Signed-off-by: Matteo Croce <teknoraver@meta.com>
Link: https://lore.kernel.org/r/20241010035652.17830-1-technoboy85@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -5371,7 +5371,7 @@ union bpf_attr {
  *		Currently, the **flags** must be 0. Currently, nr_loops is
  *		limited to 1 << 23 (~8 million) loops.
  *
- *		long (\*callback_fn)(u32 index, void \*ctx);
+ *		long (\*callback_fn)(u64 index, void \*ctx);
  *
  *		where **index** is the current index in the loop. The index
  *		is zero-indexed.

kernel/bpf/verifier.c

@@ -9917,7 +9917,7 @@ static int set_loop_callback_state(struct bpf_verifier_env *env,
 {
 	/* bpf_loop(u32 nr_loops, void *callback_fn, void *callback_ctx,
 	 *	    u64 flags);
-	 * callback_fn(u32 index, void *callback_ctx);
+	 * callback_fn(u64 index, void *callback_ctx);
 	 */
 	callee->regs[BPF_REG_1].type = SCALAR_VALUE;
 	callee->regs[BPF_REG_2] = caller->regs[BPF_REG_3];

tools/include/uapi/linux/bpf.h

@@ -5371,7 +5371,7 @@ union bpf_attr {
  *		Currently, the **flags** must be 0. Currently, nr_loops is
  *		limited to 1 << 23 (~8 million) loops.
  *
- *		long (\*callback_fn)(u32 index, void \*ctx);
+ *		long (\*callback_fn)(u64 index, void \*ctx);
  *
  *		where **index** is the current index in the loop. The index
  *		is zero-indexed.