libceph: move init of bio_iter

If a message has a non-null bio pointer, its bio_iter field is
initialized in write_partial_msg_pages() if this has not been done
already.  This is really a one-time setup operation for sending a
message's (bio) data, so move that initialization code into
prepare_write_message_data() which serves that purpose.
Signed-off-by: NAlex Elder <elder@inktank.com>
Reviewed-by: NSage Weil <sage@inktank.com>

net/ceph/messenger.c

@@ -603,6 +603,10 @@ static void prepare_write_message_data(struct ceph_connection *con)
 		con->out_msg_pos.page_pos = msg->page_alignment;
 	else
 		con->out_msg_pos.page_pos = 0;
+#ifdef CONFIG_BLOCK
+	if (msg->bio && !msg->bio_iter)
+		init_bio_iter(msg->bio, &msg->bio_iter, &msg->bio_seg);
+#endif
 	con->out_msg_pos.data_pos = 0;
 	con->out_msg_pos.did_page_crc = false;
 	con->out_more = 1;  /* data + footer will follow */
@@ -942,11 +946,6 @@ static int write_partial_msg_pages(struct ceph_connection *con)
 	     con, msg, con->out_msg_pos.page, msg->nr_pages,
 	     con->out_msg_pos.page_pos);
 
-#ifdef CONFIG_BLOCK
-	if (msg->bio && !msg->bio_iter)
-		init_bio_iter(msg->bio, &msg->bio_iter, &msg->bio_seg);
-#endif
-
 	while (data_len > con->out_msg_pos.data_pos) {
 		struct page *page = NULL;
 		int max_write = PAGE_SIZE;