rc: make the 'linux' script explicitly load filesystem modules

This removes a minor annoyance with Linux jails, where you often
want linux_mounts_enable="NO", yet you want those filesystems
available for mounting in jails; normally mount(8) would result
in kernel automatically loading the KLD, but this doesn't work
inside jails or chroots.

PR: 242955
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D29725