cifs: stop using generic_writepages

generic_writepages is just a wrapper that calls ->writepages on a range,
and thus in the way of eventually removing ->writepage.  Switch cifs
to just open code it in preparation of removing ->writepage.

[note: I suspect just integrating the small wsize case with the rest
 of the writeback code might be a better idea here, but that needs
 someone more familiar with the code]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>