Code review comment for ~goneri/cloud-init:freebsd_mount_sync

Hey Gonéri, given the above, I'm going to review this MP.

This change appears to change the default for sync from enabled to disabled for all filesystems on all non-Linux platforms. Given that the bug only reports issues with a single filesystem on a single platform, that seems like a broad change. Could we look at scoping down this change to more specifically address the issue at hand?

(Looking at https://www.freebsd.org/cgi/man.cgi?mount(8), I don't think this change is correct even just for FreeBSD; evidently some filesystems do support sync on FreeBSD.)