ext4/021: make sure the fdatasync subprocess exits

ext4/021: make sure the fdatasync subprocess exits

Now we just kill fdatasync_work process and wait nothing after the
test, so a busy unmount failure may appear if the fdatasync syscall
doesn't return in time.

  umount: /tmp/scratch: target is busy.
  mount: /tmp/scratch: /dev/sdb already mounted on /tmp/scratch.
  !!! failed to remount /dev/sdb on /tmp/scratch

This patch waits the xfs_io fdatasync subprocess exit to make sure
_check_scratch_fs success.

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>