generic: add regression test for DAX PTE/PMD races

generic: add regression test for DAX PTE/PMD races

This adds a regression test for the following kernel patches:

  mm: avoid spurious 'bad pmd' warning messages
  dax: Fix race between colliding PMD & PTE entries

The above patches fix two related PMD vs PTE races in the DAX code.
These can both be easily triggered by having two threads reading and
writing simultaneously to the same private mapping, with the key
being that private mapping reads can be handled with PMDs but
private mapping writes are always handled with PTEs so that we can
COW.

Without this 2-patch kernel series, the newly added test will result
in the following errors:

  run fstests generic/437 at 2017-05-16 16:53:43
  mm/pgtable-generic.c:39: bad pmd ffff8808daa49b88(84000001006000a5)
  	... a bunch of the bad pmd messages ...
  BUG: Bad rss-counter state mm:ffff8800a8c1b700 idx:1 val:1
  BUG: non-zero nr_ptes on freeing mm: 38
  XFS (pmem0p1): Unmounting Filesystem

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>