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>