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/file.c

@@ -1648,6 +1648,27 @@ static int cifs_partialpagewrite(struct page *page, unsigned from, unsigned to)
 	return rc;
 }
 
+/*
+ * Marshal up the iov array, reserving the first one for the header. Also,
+ * set wdata->bytes.
+ */
+static void
+cifs_writepages_marshal_iov(struct kvec *iov, struct cifs_writedata *wdata)
+{
+	int i;
+	struct inode *inode = wdata->cfile->dentry->d_inode;
+	loff_t size = i_size_read(inode);
+
+	/* marshal up the pages into iov array */
+	wdata->bytes = 0;
+	for (i = 0; i < wdata->nr_pages; i++) {
+		iov[i + 1].iov_len = min(size - page_offset(wdata->pages[i]),
+					(loff_t)PAGE_CACHE_SIZE);
+		iov[i + 1].iov_base = kmap(wdata->pages[i]);
+		wdata->bytes += iov[i + 1].iov_len;
+	}
+}
+
 static int cifs_writepages(struct address_space *mapping,
 			   struct writeback_control *wbc)
 {
@@ -1792,6 +1813,7 @@ retry:
 		wdata->sync_mode = wbc->sync_mode;
 		wdata->nr_pages = nr_pages;
 		wdata->offset = page_offset(wdata->pages[0]);
+		wdata->marshal_iov = cifs_writepages_marshal_iov;
 
 		do {
 			if (wdata->cfile != NULL)