cleanvar: Fix startup order

Instead of having FILESYSTEMS require cleanvar, which doesn't really
make semantic sense, say that cleanvar needs to run before FILESYSTEMS.

MFC after: 3 days
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D54118

(cherry picked from commit 6ce227d6274869a95150746d2f2d8c8c5ed9a266)