ZTS: Speed up write_dirs cleanup

The write_dirs tests fill a filesystem with a bunch of files until it
is full.  In cleanup the files are truncated and removed individually.
These tests already take a while to run.

It is quicker and easier to destroy the whole dataset and create a new
one to replace it in the cleanup functions.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10098

tests/zfs-tests/tests/functional/write_dirs/write_dirs_001_pos.ksh

@@ -47,11 +47,8 @@ verify_runnable "both"
 
 function cleanup
 {
-	for file in `find $TESTDIR -type f`; do
-		cat /dev/null > $file
-	done
-	log_must sync
-	log_must rm -rf $TESTDIR/*
+	destroy_dataset $TESTPOOL/$TESTFS
+	zfs create -o mountpoint=$TESTDIR $TESTPOOL/$TESTFS
 }
 
 typeset -i retval=0

tests/zfs-tests/tests/functional/write_dirs/write_dirs_002_pos.ksh

@@ -47,11 +47,8 @@ verify_runnable "both"
 
 function cleanup
 {
-	for file in `find $TESTDIR -type f`; do
-		cat /dev/null > $file
-	done
-	log_must sync
-	log_must rm -rf $TESTDIR/*
+	destroy_dataset $TESTPOOL/$TESTFS
+	zfs create -o mountpoint=$TESTDIR $TESTPOOL/$TESTFS
 }
 
 typeset -i retval=0