btrfs/016: fix a false alert due to xattrs mismatch

btrfs/016: fix a false alert due to xattrs mismatch

[BUG]
When running btrfs/016 after any other test case, it would fail on a
SELinux enabled environment:

  btrfs/015 1s ...  0s
  btrfs/016 1s ... [failed, exit status 1]- output mismatch (see ~/xfstests-dev/results//btrfs/016.out.bad)
      --- tests/btrfs/016.out	2023-12-28 10:39:36.481027970 +1030
      +++ ~/xfstests-dev/results//btrfs/016.out.bad	2023-12-28 15:53:10.745436664 +1030
      @@ -1,2 +1,3 @@
       QA output created by 016
      -Silence is golden
      +fssum failed
      +(see ~/xfstests-dev/results//btrfs/016.full for details)
      ...
      (Run 'diff -u ~/xfstests-dev/tests/btrfs/016.out ~/xfstests-dev/results//btrfs/016.out.bad'  to see the entire diff)
  Ran: btrfs/015 btrfs/016
  Failures: btrfs/016
  Failed 1 of 2 tests

[CAUSE]
The test case itself would try to use a blank SELinux context for the
SCRATCH_MNT, to control the xattrs.

But the initial send stream is generated from $TEST_DIR, which may still
have the default SELinux mount context.

And such mismatch in the SELinux xattr (source on $TEST_DIR still has
the extra xattr, meanwhile the receve end on $SCRATCH_MNT doesn't) would
lead to above mismatch.

[FIX]
Fix the false alerts by disable XATTR checks.

Furthermore instead of doing all the edge juggling using $TEST_DIR, this
time we do all the work on $SCRATCH_MNT.

This means we would generate the initial send stream from $SCRATCH_MNT,
then reformat the fs, mount scratch again, receive and verify.

We no longer needs to cleanup the extra file for the initial send
stream, as they are on the scratch device and would be formatted anyway.

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