x86/ioperm: Avoid bitmap allocation if no permissions are set

If ioperm() is invoked the first time and the @turn_on argument is 0, then
there is no point to allocate a bitmap just to clear permissions which are
not set.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/ioport.c

@@ -36,6 +36,9 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)
 	 */
 	bitmap = t->io_bitmap_ptr;
 	if (!bitmap) {
+		/* No point to allocate a bitmap just to clear permissions */
+		if (!turn_on)
+			return 0;
 		bitmap = kmalloc(IO_BITMAP_BYTES, GFP_KERNEL);
 		if (!bitmap)
 			return -ENOMEM;