dm-raid: fix variable in journal device check

Replace "rdev" with correct loop variable name "r".

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 63c32ed4 ("dm raid: add raid4/5/6 journaling support")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>