[PATCH] fstests: btrfs/253: fix false alert due to _set_fs_sysfs_attr changes

[Qu Wenruo <wqu@suse.com>]

[FALSE FAILURE]
Test btrfs/253 now fails like the following:

btrfs/253 2s ... - output mismatch (see ~/xfstests/results//btrfs/253.out.bad)
    --- tests/btrfs/253.out	2022-05-11 11:25:30.753333331 +0930
    +++ ~/xfstests/results//btrfs/253.out.bad	2025-04-20 17:28:39.139180394 +0930
    @@ -5,6 +5,7 @@
     Calculate request size so last memory allocation cannot be completely fullfilled.
     Third allocation.
     Force allocation of system block type must fail.
    +./common/rc: line 5213: echo: write error: No space left on device
     Verify first allocation.
     Verify second allocation.
     Verify third allocation.
    ...
    (Run 'diff -u ~/xfstests/tests/btrfs/253.out ~/xfstests/results//btrfs/253.out.bad'  to see the entire diff)

[CAUSE]
Since commit 0a9011ae6a36 ("fstests: common/rc: set_fs_sysfs_attr:
redirect errors to stdout") the function _set_fs_sysfs_attr() always
output everything into stdout, thus the stderr redirection makes no
sense anymore.

And the expected failure will cause output difference and fail the test.

[FIX]
Instead of the useless re-direct of stderr, save the stdout and check if
it contains the word "error" to determine if it failed.

Fixes: 0a9011ae6a36 ("fstests: common/rc: set_fs_sysfs_attr: redirect errors to stdout")
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 tests/btrfs/253 | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/tests/btrfs/253 b/tests/btrfs/253
index adbc6bfb..ad69dfe1 100755
--- a/tests/btrfs/253
+++ b/tests/btrfs/253
@@ -228,7 +228,12 @@ alloc_size "Data" FOURTH_DATA_SIZE_MB
 # Force chunk allocation of system block type must fail.
 #
 echo "Force allocation of system block type must fail."
-_set_fs_sysfs_attr ${SCRATCH_BDEV} allocation/system/force_chunk_alloc 1 2>/dev/null
+output=$(_set_fs_sysfs_attr ${SCRATCH_BDEV} allocation/system/force_chunk_alloc 1)
+
+if ! echo "$output" | grep -q "error" ; then
+	echo "Force allocation succeeded unexpectedly."
+	echo "$output" >> $seqres.full
+fi
 
 #
 # Verification of initial allocation.
-- 
2.47.1