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>