ZTS: Only use ext4 on Linux

And while here, add a workaround for FreeBSD to ensure dirty data is
synced before taking a snapshot, by remounting read-only, then remount
again read-write after the snapshot.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #9914