btrfs/254: fix test failure due to already unmounted scratch device

btrfs/254: fix test failure due to already unmounted scratch device

If there are no failures in the middle of test while the 3rd scratch
device is mounted (at $seq_mnt), the unmount call in the _cleanup
function will result in a test failure since the unmount already
happened, making the test fail:

  $ ./check btrfs/254
  FSTYP         -- btrfs
  PLATFORM      -- Linux/x86_64 debian0 6.14.0-rc4-btrfs-next-188+ #1 SMP PREEMPT_DYNAMIC Wed Feb 26 17:38:41 WET 2025
  MKFS_OPTIONS  -- /dev/sdc
  MOUNT_OPTIONS -- /dev/sdc /home/fdmanana/btrfs-tests/scratch_1

  btrfs/254 2s ... - output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/254.out.bad)
      --- tests/btrfs/254.out	2024-10-07 12:36:15.532225987 +0100
      +++ /home/fdmanana/git/hub/xfstests/results//btrfs/254.out.bad	2025-02-27 12:53:30.848728429 +0000
      @@ -3,3 +3,4 @@
       	Total devices <NUM> FS bytes used <SIZE>
       	devid <DEVID> size <SIZE> used <SIZE> path SCRATCH_DEV
       	*** Some devices missing
      +umount: /home/fdmanana/btrfs-tests/dev/254.mnt: not mounted.
      ...
      (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/254.out /home/fdmanana/git/hub/xfstests/results//btrfs/254.out.bad'  to see the entire diff)

  HINT: You _MAY_ be missing kernel fix:
        770c79fb6550 btrfs: harden identification of a stale device

  Ran: btrfs/254
  Failures: btrfs/254
  Failed 1 of 1 tests

This is a recent regression because the _unmount function used to redirect
stdout and stderr to $seqres.full, but not anymore since the recent commit
identified in the Fixes tag below. So redirect stdout and stderr of the
call to _unmount in the _cleanup function to /dev/null.

Fixes: f43da58ef936 ("unmount: resume logging of stdout and stderr for filtering")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>