ext4: test for DAX + journaling corruption

ext4: test for DAX + journaling corruption

Add a regression test for the following kernel commit:

  ext4: prevent data corruption with journaling + DAX

The test passes if either we successfully compare the data between
the mmap with journaling turned on and the one with journaling
turned off, or if we fail the chattr command to turn on or off
journaling.  The latter is how we prevent this issue in the kernel.

[eguan: add 'dax' group]

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>