btrfs/132: wait the writer to avoid EBUSY

The kill and wait method will only wait for the children process to
exit, while the xfs_io can still run at background.

This makes the test always fails on HDD backed physical machine.

Use the "while ps aux | grep" method in btrfs/069 to truely wait the
xfs_io to finish.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/btrfs/132

@@ -94,9 +94,10 @@ sleep $sleep_time
 kill $pids
 wait
 
-# Sync the fs to avoid EBUSY while umount, which is quite common for btrfs
-# compression
-sync
+# Wait all writers really exits
+while ps aux | grep "$SCRATCH_MNT" | grep -qv grep; do
+	sleep 1
+done
 
 echo "Silence is golden"
 status=0