hfsplus: simplify fsync

Remove lots of code we don't need from fsync, we just need to call
->write_inode on the inode if it's dirty, for which sync_inode_metadata
is a lot more efficient than write_inode_now, and we need to write
out the various metadata inodes, which we now do explicitly instead
of by calling ->sync_fs.

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