btrfs/011: Fill the fs to ensure we have enough data for dev-replace

btrfs/011: Fill the fs to ensure we have enough data for dev-replace

[BUG]
When btrfs/011 is executed on a fast enough system (fully memory backed
VM, with test device has unsafe cache mode), the test can fail like
this:

  btrfs/011 43s ... [failed, exit status 1]- output mismatch (see /home/adam/xfstests-dev/results//btrfs/011.out.bad)
    --- tests/btrfs/011.out     2019-07-22 14:13:44.643333326 +0800
    +++ /home/adam/xfstests-dev/results//btrfs/011.out.bad      2019-09-18 14:49:28.308798022 +0800
    @@ -1,3 +1,4 @@
     QA output created by 011
     *** test btrfs replace
    -*** done
    +failed: '/usr/bin/btrfs replace cancel /mnt/scratch'
    +(see /home/adam/xfstests-dev/results//btrfs/011.full for details)
    ...

[CAUSE]
Looking into the full output, it shows:
  ...
  Replace from /dev/mapper/test-scratch1 to /dev/mapper/test-scratch2

  # /usr/bin/btrfs replace start -f /dev/mapper/test-scratch1 /dev/mapper/test-scratch2 /mnt/scratch
  # /usr/bin/btrfs replace cancel /mnt/scratch
  INFO: ioctl(DEV_REPLACE_CANCEL)"/mnt/scratch": not started
  failed: '/usr/bin/btrfs replace cancel /mnt/scratch'

So this means the replace is already finished before we cancel it.
For fast system, it's very common.

[FIX]
In fill_scratch() after all the original file creations, do a timer
based direct IO write.
The extra write will take 2 * $wait_time, utilizing direct IO with 64K
block size, the write performance should be very comparable (although a
little faster) to replace performance.

So later cancel should be able to really cancel the dev-replace without
it finished too early.

Also, do extra check about the above write. If we hit ENOSPC we just
skip the test as the system is really too fast and the fs is not large
enough.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>