x86/ioperm: Add bitmap sequence number

Add a globally unique sequence number which is incremented when ioperm() is
changing the I/O bitmap of a task. Store the new sequence number in the
io_bitmap structure and compare it with the sequence number of the I/O
bitmap which was last loaded on a CPU. Only update the bitmap if the
sequence is different.

That should further reduce the overhead of I/O bitmap scheduling when there
are only a few I/O bitmap users on the system.

The 64bit sequence counter is sufficient. A wraparound of the sequence
counter assuming an ioperm() call every nanosecond would require about 584
years of uptime.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/include/asm/processor.h

@@ -361,6 +361,9 @@ struct entry_stack_page {
  * All IO bitmap related data stored in the TSS:
  */
 struct x86_io_bitmap {
+	/* The sequence number of the last active bitmap. */
+	u64			prev_sequence;
+
 	/*
 	 * Store the dirty size of the last io bitmap offender. The next
 	 * one will have to do the cleanup as the switch out to a non io