rc: improve dependencies for growfs

Previously it depended on sysctl, which itself has no dependencies,
so rcorder(8) had a bit too much flexibility when choosing when to run
it. Make sure it runs just between 'fsck' and 'root'.

Reviewed By: jmg, imp
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D29748

(cherry picked from commit b68e6569221ecade8899eaab8e1b088947ef2f8a)