selftests/bpf: Fix tests after fields reorder in struct file

The change in struct file [1] moved f_ref to the 3rd cache line.
It made *(u64 *)file dereference invalid from the verifier point of view,
because btf_struct_walk() walks into f_lock field, which is 4-byte long.

Fix the selftests to deference the file pointer as a 4-byte access.

[1] commit e249056 ("fs: place f_ref to 3rd cache line in struct file to resolve false sharing")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250327185528.1740787-1-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: liu-song-6

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

@@ -117,7 +117,7 @@ int BPF_PROG(handle_fexit_ret, int arg, struct file *ret)
 
 	bpf_probe_read_kernel(&buf, 8, ret);
 	bpf_probe_read_kernel(&buf, 8, (char *)ret + 256);
-	*(volatile long long *)ret;
+	*(volatile int *)ret;
 	*(volatile int *)&ret->f_mode;
 	return 0;
 }

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

@@ -21,7 +21,7 @@ static __u64 test_cb(struct bpf_map *map, __u32 *key, __u64 *val, void *data)
 SEC("fexit/bpf_testmod_return_ptr")
 int BPF_PROG(handle_fexit_ret_subprogs, int arg, struct file *ret)
 {
-	*(volatile long *)ret;
+	*(volatile int *)ret;
 	*(volatile int *)&ret->f_mode;
 	bpf_for_each_map_elem(&test_array, test_cb, NULL, 0);
 	triggered++;
@@ -31,7 +31,7 @@ int BPF_PROG(handle_fexit_ret_subprogs, int arg, struct file *ret)
 SEC("fexit/bpf_testmod_return_ptr")
 int BPF_PROG(handle_fexit_ret_subprogs2, int arg, struct file *ret)
 {
-	*(volatile long *)ret;
+	*(volatile int *)ret;
 	*(volatile int *)&ret->f_mode;
 	bpf_for_each_map_elem(&test_array, test_cb, NULL, 0);
 	triggered++;
@@ -41,7 +41,7 @@ int BPF_PROG(handle_fexit_ret_subprogs2, int arg, struct file *ret)
 SEC("fexit/bpf_testmod_return_ptr")
 int BPF_PROG(handle_fexit_ret_subprogs3, int arg, struct file *ret)
 {
-	*(volatile long *)ret;
+	*(volatile int *)ret;
 	*(volatile int *)&ret->f_mode;
 	bpf_for_each_map_elem(&test_array, test_cb, NULL, 0);
 	triggered++;