autofs: media: Always use sync option for fat*

Users of autofs for removable media expect to be able to copy files and
directly remove the media without having the need to call sync(8) or umount(8).
Only do that for fat/ntfs filesystems.

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D42494
Reviewed by: rew (older version)

(cherry picked from commit 094abb6fb41c07c0266a5cae84a7439289a978e9)