hfsplus: flush disk caches in sync and fsync

Flush the disk cache in fsync and sync to make sure data actually is
on disk on completion of these system calls.  There is a nobarrier
mount option to disable this behaviour.  It's slightly misnamed now
that barrier actually are gone, but it matches the name used by all
major filesystems.

Signed-off-by: Christoph Hellwig <hch@tuxera.com>