ceph: use calc_pages_for() in start_read()

There's a spot that computes the number of pages to allocate for a
page-aligned length by just shifting it.  Use calc_pages_for()
instead, to be consistent with usage everywhere else.  The result
is the same.

The reason for this is to make it clearer in an upcoming patch that
this calculation is duplicated.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

fs/ceph/addr.c

@@ -314,7 +314,7 @@ static int start_read(struct inode *inode, struct list_head *page_list, int max)
 		return PTR_ERR(req);
 
 	/* build page vector */
-	nr_pages = len >> PAGE_CACHE_SHIFT;
+	nr_pages = calc_pages_for(0, len);
 	pages = kmalloc(sizeof(*pages) * nr_pages, GFP_NOFS);
 	ret = -ENOMEM;
 	if (!pages)