mm,vmacache: count number of system-wide flushes

These flushes deal with sequence number overflows, such as for long lived
threads.  These are rare, but interesting from a debugging PoV.  As such,
display the number of flushes when vmacache debugging is enabled.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmacache.c

@@ -17,6 +17,8 @@ void vmacache_flush_all(struct mm_struct *mm)
 {
 	struct task_struct *g, *p;
 
+	count_vm_vmacache_event(VMACACHE_FULL_FLUSHES);
+
 	/*
 	 * Single threaded tasks need not iterate the entire
 	 * list of process. We can avoid the flushing as well