selftests/bpf: Change variable types for -Wsign-compare

This is a follow up patch for commit 495d2d8("selftests/bpf: Attempt
to build BPF programs with -Wsign-compare") from Alexei Starovoitov[1]
to be able to enable -Wsign-compare C compilation flag for clang since
-Wall doesn't add it and BPF programs are built with clang.This has the
benefit to catch problematic comparisons in future tests as quoted from
the commit message:"
  int i = -1;
  unsigned int j = 1;
  if (i < j) // this is false.

  long i = -1;
  unsigned int j = 1;
  if (i < j) // this is true.

C standard for reference:

- If either operand is unsigned long the other shall be converted to
unsigned long.

- Otherwise, if one operand is a long int and the other unsigned int,
then if a long int can represent all the values of an unsigned int,
the unsigned int shall be converted to a long int;
otherwise both operands shall be converted to unsigned long int.

- Otherwise, if either operand is long, the other shall be
converted to long.

- Otherwise, if either operand is unsigned, the other shall be
converted to unsigned.

Unfortunately clang's -Wsign-compare is very noisy.
It complains about (s32)a == (u32)b which is safe and doen't
have surprising behavior."

This specific patch supresses the following warnings when
-Wsign-compare is enabled:

1 warning generated.

progs/bpf_iter_bpf_percpu_array_map.c:35:16: warning: comparison of
integers of different signs: 'int' and 'const volatile __u32'
(aka 'const volatile unsigned int') [-Wsign-compare]
   35 |         for (i = 0; i < num_cpus; i++) {
      |                     ~ ^ ~~~~~~~~

1 warning generated.

progs/bpf_qdisc_fifo.c:93:2: warning: comparison of integers of
different signs: 'int' and '__u32'
(aka 'unsigned int') [-Wsign-compare]
   93 |         bpf_for(i, 0, sch->q.qlen) {
      |         ^       ~     ~~~~~~~~~~~

Should be noted that many more similar changes are still needed in order
to be able to enable the -Wsign-compare flag since -Werror is enabled and
would cause compilation of bpf selftests to fail.

[1].
Link:torvalds/linux@495d2d8

Signed-off-by: Mehdi Ben Hadj Khelifa <[email protected]>

Author: Mehdi Ben Hadj Khelifa

tools/testing/selftests/bpf/progs/bpf_iter_bpf_percpu_array_map.c

@@ -24,7 +24,7 @@ int dump_bpf_percpu_array_map(struct bpf_iter__bpf_map_elem *ctx)
 	__u32 *key = ctx->key;
 	void *pptr = ctx->value;
 	__u32 step;
-	int i;
+	__u32 i;
 
 	if (key == (void *)0 || pptr == (void *)0)
 		return 0;

tools/testing/selftests/bpf/progs/bpf_qdisc_fifo.c

@@ -88,7 +88,7 @@ void BPF_PROG(bpf_fifo_reset, struct Qdisc *sch)
 {
 	struct bpf_list_node *node;
 	struct skb_node *skbn;
-	int i;
+	__u32 i;
 
 	bpf_for(i, 0, sch->q.qlen) {
 		struct sk_buff *skb = NULL;