selftests/bpf: Fix bpf selftest build error

On linux-next, build for bpf selftest displays an error due to
mismatch in the expected function signature of bpf_testmod_test_read
and bpf_testmod_test_write.

Commit 97d0680 ("sysfs: constify bin_attribute argument of bin_attribute::read/write()")
changed the required type for struct bin_attribute to const struct bin_attribute.

To resolve the error, update corresponding signature for the callback.

Fixes: 97d0680 ("sysfs: constify bin_attribute argument of bin_attribute::read/write()")
Reported-by: Venkat Rao Bagalkote <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Tested-by: Venkat Rao Bagalkote <[email protected]>
Signed-off-by: Saket Kumar Bhaskar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Saket Kumar Bhaskar

tools/testing/selftests/bpf/test_kmods/bpf_testmod.c

@@ -385,7 +385,7 @@ int bpf_testmod_fentry_ok;
 
 noinline ssize_t
 bpf_testmod_test_read(struct file *file, struct kobject *kobj,
-		      struct bin_attribute *bin_attr,
+		      const struct bin_attribute *bin_attr,
 		      char *buf, loff_t off, size_t len)
 {
 	struct bpf_testmod_test_read_ctx ctx = {
@@ -465,7 +465,7 @@ ALLOW_ERROR_INJECTION(bpf_testmod_test_read, ERRNO);
 
 noinline ssize_t
 bpf_testmod_test_write(struct file *file, struct kobject *kobj,
-		      struct bin_attribute *bin_attr,
+		      const struct bin_attribute *bin_attr,
 		      char *buf, loff_t off, size_t len)
 {
 	struct bpf_testmod_test_write_ctx ctx = {
@@ -567,7 +567,7 @@ static void testmod_unregister_uprobe(void)
 
 static ssize_t
 bpf_testmod_uprobe_write(struct file *file, struct kobject *kobj,
-			 struct bin_attribute *bin_attr,
+			 const struct bin_attribute *bin_attr,
 			 char *buf, loff_t off, size_t len)
 {
 	unsigned long offset = 0;