dm: lock limits when reading them

commit abb4cf2f upstream.

Lock queue limits when reading them, so that we don't read halfway
modified values.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>