msdosfs: Don't #ifdef out msdosfsmount members

Whilst these aren't used by makefs, they do little harm existing once
the needed headers are included, and having structs change layout based
on defines like this can be fraught. This will be particularly true once
this code is exposed by defines other than _KERNEL and MAKEFS, as any
consumer will be able to opt into exposing this kernel type and all the
definitions should match.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50885