msdosfs: suspend around unmount or remount rw->ro.

This also eliminates unsafe use of VFS_SYNC(MNT_WAIT).

Requested by: mckusick
Discussed with: imp
Tested by: pho (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D27269