fstests: add support for working with dm-log-writes target

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

  This patch adds the supporting code for using the dm-log-writes
  target.  The dmlogwrites code is similar to the dmflakey code, it just
  gives us functions to build and tear down a dm-log-writes target.  We
  add a new LOGWRITES_DEV variable to take in the device we will use as
  the log and add checks for that.

[Amir:]
- Removed unneeded _test_falloc_support
- Moved _require_log_writes to dmlogwrites
- Document _require_log_writes
- Address review comments by Eryu Guan

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>

doc/requirement-checking.txt

@@ -21,6 +21,10 @@ they have.  This is done with _require_<xxx> macros, which may take parameters.
 
 	_require_statx
 
+ (4) Device mapper requirement.
+
+	_require_dm_target
+	_require_log_writes
 
 ====================
 GENERAL REQUIREMENTS
@@ -102,3 +106,19 @@ _require_statx
 
      The test requires the use of the statx() system call and will be skipped
      if it isn't available in the kernel.
+
+
+==========================
+DEVICE MAPPER REQUIREMENTS
+==========================
+
+_require_dm_target <name>
+
+     The test requires the use of the device mapper target and will be skipped
+     if it isn't available in the kernel.
+
+_require_log_writes
+
+     The test requires the use of the device mapper target log-writes.
+     The test also requires the test program log-writes/replay-log is built
+     and will be skipped if either isn't available.