[PATCH] fstests: btrfs/340: add support for older kernels/progs

The test case will fail on older distros like SLE12-SP5, there are two
problems on the test case itself:

- Requires "--sync" option from "btrfs qgroup show" subcommand
  That option was introduecd in v4.9.1, thus there are still some older
  distros using that very old version.

- Not all kernels support quick inherit
  The kernel commit introducing the quick inherit is 6.9, thus some
  supported LTS kernels do not have that commit.

  In that case qgroup will just be marked inconsistent, this will cause
  the later "btrfs qgroup show" command to output a warning due to the
  inconsistent flag.

Fix both problems by:

- Require "--sync" option from "btrfs qgroup show" subcommand
  So older distros will just skip the test.

- Redirect all output from "btrfs qgroup show" to $seqres.full
  So older kernels can still pass the test case by marking qgroup
  inconsitent.

Reported-by: Long An <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>
---
 tests/btrfs/340 | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/tests/btrfs/340 b/tests/btrfs/340
index 872f790cb975..fc0b8d48adaa 100755
--- a/tests/btrfs/340
+++ b/tests/btrfs/340
@@ -39,7 +39,14 @@ _btrfs qgroup show -p --sync $SCRATCH_MNT >> $seqres.full
 # without marking qgroup inconsistent.
 _btrfs subv snap -i 1/1 $SCRATCH_MNT/subv1 $SCRATCH_MNT/snap1
 echo "After quick inherit:" >> $seqres.full
-_btrfs qgroup show -p --sync $SCRATCH_MNT >> $seqres.full
+# Sync the fs so that the qgroup numbers are properly updated.
+_btrfs filesystem sync $SCRATCH_MNT
+
+# Redirect all output to seqres.full including stderr.
+# For older kernels without quick inherit support, it will mark
+# qgroup inconsistent, which can pollute the golden output.
+# Here we rely on the final btrfs check to catch any qgroup number errors.
+_btrfs qgroup show -p $SCRATCH_MNT >> $seqres.full
 
 echo "Silence is golden"
 _exit 0
-- 
2.54.0