Add a bugcheck for Andrea's pagefault vs invalidate race.  This is triggerable
for both linear and nonlinear pages with a userspace test harness (using
direct IO and truncate, respectively).
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>