xfstests 285: Fix test for ext4 in some configurations

In some configurations (e.g. 1 KB block size), ext4 can decide it is
better to zero out several blocks rather than splitting unwritten
extent. This changes results SEEK_HOLE / SEEK_DATA returns and thus the
test fails. Fix the problem by disabling the feature for this test.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>

tests/generic/285

@@ -46,6 +46,12 @@ BASE_TEST_FILE=$TEST_DIR/seek_sanity_testfile
 
 [ -x $here/src/seek_sanity_test ] || _notrun "seek_sanitfy_tester not built"
 
+# Disable extent zeroing for ext4 as that change where holes are created
+if [ "$FSTYP" = "ext4" ]; then
+	DEV=`basename $TEST_DEV`
+	echo 0 >/sys/fs/ext4/$DEV/extent_max_zeroout_kb
+fi
+
 _cleanup()
 {
 	eval "rm -f $BASE_TEST_FILE.*"