Fix ZFS ARC min / max tunable

Due to ARC initial configuration not being done and kmem information
not being available we need to blindly set zfs_arc_max and zfs_arc_min
when configured via the tunable.

This fixes vfs.zfs.arc_(min|max) configuration via loader.conf broken by
r302265.

Approved by: re(gjb)
MFC after: 1 week