generic: crash consistency fsx test using dm-log-writes

generic: crash consistency fsx test using dm-log-writes

Cherry-picked the test from commit 70d41e17164b
in Josef Bacik's fstests tree (https://github.com/josefbacik/fstests).
Quoting from Josef's commit message:

  The test just runs some ops and exits, then finds all of the good buffers
  in the directory we provided and:
  - replays up to the mark given
  - mounts the file system and compares the md5sum
  - unmounts and fsck's to check for metadata integrity

  dm-log-writes will pretend to do discard and the replay-log tool will
  replay it properly depending on the underlying device, either by writing
  0's or actually calling the discard ioctl, so I've enabled discard in the
  test for maximum fun.

[Amir:]
- Removed unneeded _test_falloc_support dynamic FSX_OPTS
- Fold repetitions into for loops
- Added place holders for using constant random seeds
- Add pre umount checkpint
- Add test to new 'replay' group
- Address review comments by Eryu Guan

[eguan: fixed minor code style issues, remove extra newline at eof]

Cc: Josef Bacik <jbacik@fb.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>