dax: Implement dax_finish_sync_fault()

Implement a function that filesystems can call to finish handling of
synchronous page faults. It takes care of syncing appropriare file range
and insertion of page table entry.

Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>