[PATCH v3] common/rc: Modify _require_batched_discard to improve test coverage

[Ojaswin Mujoo <ojaswin@linux.ibm.com>]

A recent ext4 patch discussed [1] that some devices (eg LVMs) can
have a discard granularity as big as 42MB which makes it larger
than the group size of ext4 FS with 1k BS.  This causes the FITRIM
IOCTL to fail.

This case was not correctly handled by this test since
"_require_batched_discard" incorrectly interpreted the FITRIM
failure as SCRATCH_DEV not supporting the IOCTL. This caused the test
to report "not run" instead of "failed" in case of large discard granularity.

Fix "_require_batched_discard" to use a more accurate method
to determine if discard is supported.

[1] commit 173b6e383d2
    ext4: avoid trim error on fs with small groups

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
---

Changes since v2 [1] 

*  Eliminated redundant $ret variable

[1]
https://lore.kernel.org/all/20220516063951.87838-1-ojaswin@linux.ibm.com/

 common/rc | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/common/rc b/common/rc
index e2d3d72a..94344248 100644
--- a/common/rc
+++ b/common/rc
@@ -3858,7 +3858,12 @@ _require_batched_discard()
 		exit 1
 	fi
 	_require_fstrim
-	$FSTRIM_PROG $1 > /dev/null 2>&1 || _notrun "FITRIM not supported on $1"
+
+	grep -q "not supported" <($FSTRIM_PROG $1 2>&1)
+	if [ "$?" = "0" ]
+	then
+		_notrun "FITRIM not supported on $1"
+	fi
 }
 
 _require_dumpe2fs()
-- 
2.27.0