cifs: abstract out function to marshal the iovec for readv receives

Cached and uncached reads will need to do different things here to
handle the difference when the pages are in pagecache and not. Abstract
out the function that marshals the page list into a kvec array.

Signed-off-by: Jeff Layton <jlayton@redhat.com>

fs/cifs/cifsproto.h

@@ -472,6 +472,8 @@ struct cifs_readdata {
 	int				result;
 	struct list_head		pages;
 	struct work_struct		work;
+	int (*marshal_iov) (struct cifs_readdata *rdata,
+			    unsigned int remaining);
 	unsigned int			nr_iov;
 	struct kvec			iov[1];
 };