btrfs/213: avoid occasional failure due to already finished balance

btrfs/213: avoid occasional failure due to already finished balance

btrfs/213 writes data, in 1M extents, for 4 seconds into a file, then
triggers a balance and then after 2 seconds it tries to cancel the
balance operation. More often than not, this works because the balance
is still running after 2 seconds. However it also fails sporadically
because balance has finished in less than 2 seconds, which is plausible
since data and metadata are cached or other factors such as virtualized
environment. When that's the case, it fails like this:

  $ ./check btrfs/213
  FSTYP         -- btrfs
  PLATFORM      -- Linux/x86_64 debian0 6.4.0-rc1-btrfs-next-131+ #1 SMP PREEMPT_DYNAMIC Thu May 11 11:26:19 WEST 2023
  MKFS_OPTIONS  -- /dev/sdc
  MOUNT_OPTIONS -- /dev/sdc /home/fdmanana/btrfs-tests/scratch_1

  btrfs/213 51s ... - output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/213.out.bad)
      --- tests/btrfs/213.out	2020-06-10 19:29:03.822519250 +0100
      +++ /home/fdmanana/git/hub/xfstests/results//btrfs/213.out.bad	2023-05-17 15:39:32.653727223 +0100
      @@ -1,2 +1,3 @@
       QA output created by 213
      +ERROR: balance cancel on '/home/fdmanana/btrfs-tests/scratch_1' failed: Not in progress
       Silence is golden
      ...
      (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/213.out /home/fdmanana/git/hub/xfstests/results//btrfs/213.out.bad'  to see the entire diff)
  Ran: btrfs/213
  Failures: btrfs/213
  Failed 1 of 1 tests

To make it much less likely that balance has already finished before we
try to cancel it, unmount and mount again the filesystem before starting
balance, to clear cached metadata and data, and also double the time we
spend writing 1M data extents. Also make the test not run with an
informative message if we detect that balance finished before we could
cancel it.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>