hfsplus: make sure sync writes out all metadata

hfsplus stores all metadata except for the volume headers in special
inodes.  While these are marked hashed and periodically written out
by the flusher threads, we can't rely on that for sync.  For the case
of a data integrity sync the VM has life-lock avoidance code that
avoids writing inodes again that are redirtied during the sync,
which is something that can happen easily for hfsplus.  So make sure
we explicitly write out the metadata inodes at the beginning of
hfsplus_sync_fs.

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