[PATCH] 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 on filesystems like ext4.

This case was not correctly handle by "_require_batched_discard" as
it incorrectly interpreted the FITRIM failure as fs not supporting
the IOCTL. This caused the tests like generic/260 to incorectly
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>
---
 common/rc | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/common/rc b/common/rc
index e2d3d72a..97386342 100644
--- a/common/rc
+++ b/common/rc
@@ -3858,7 +3858,13 @@ _require_batched_discard()
 		exit 1
 	fi
 	_require_fstrim
-	$FSTRIM_PROG $1 > /dev/null 2>&1 || _notrun "FITRIM not supported on $1"
+
+	$FSTRIM_PROG $1 2>&1 | grep -q "not supported"
+	RET=$?
+	if [ "$RET" = "0" ]
+	then
+		_notrun "FITRIM not supported on $1"
+	fi
 }
 
 _require_dumpe2fs()
-- 
2.27.0