mm/kmemleak.c: change error at _write when kmemleak is disabled

According to POSIX, EBUSY means that the "device or resource is busy", and
this can lead to people thinking that the file
`/sys/kernel/debug/kmemleak/` is somehow locked or being used by other
process.  Change this error code to a more appropriate one.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: André Almeida <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/kmemleak.c

@@ -1866,7 +1866,7 @@ static ssize_t kmemleak_write(struct file *file, const char __user *user_buf,
 	}
 
 	if (!kmemleak_enabled) {
-		ret = -EBUSY;
+		ret = -EPERM;
 		goto out;
 	}