iotests: Don't lock /dev/null in 226

On my system (Fedora 28), this script reports a 'failed to get
"consistent read" lock' error. Following docs/devel/testing.rst, it's
better to add locking=off here.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/226

@@ -56,10 +56,10 @@ for PROTO in "file" "host_device" "host_cdrom"; do
     echo
     echo "== Testing RO =="
     $QEMU_IO -c "open -r -o driver=$PROTO,filename=$TEST_IMG" 2>&1 | _filter_testdir | _filter_imgfmt
-    $QEMU_IO -c "open -r -o driver=$PROTO,filename=/dev/null" 2>&1 | _filter_imgfmt
+    $QEMU_IO -c "open -r -o driver=$PROTO,filename=/dev/null,locking=off" 2>&1 | _filter_imgfmt
     echo "== Testing RW =="
     $QEMU_IO -c "open -o driver=$PROTO,filename=$TEST_IMG" 2>&1 | _filter_testdir | _filter_imgfmt
-    $QEMU_IO -c "open -o driver=$PROTO,filename=/dev/null" 2>&1 | _filter_imgfmt
+    $QEMU_IO -c "open -o driver=$PROTO,filename=/dev/null,locking=off" 2>&1 | _filter_imgfmt
 done
 
 # success, all done