cifs: abstract out function to marshal up the iovec array for async writes

We'll need to do something a bit different depending on the caller.
Abstract the code that marshals the page array into an iovec.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>

fs/cifs/cifsproto.h

@@ -490,6 +490,8 @@ struct cifs_writedata {
 	pid_t				pid;
 	unsigned int			bytes;
 	int				result;
+	void (*marshal_iov) (struct kvec *iov,
+			     struct cifs_writedata *wdata);
 	unsigned int			nr_pages;
 	struct page			*pages[1];
 };