cifs: fix up get_numpages

Use DIV_ROUND_UP. Also, PAGE_SIZE is more appropriate here since these
aren't pagecache pages.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 70bd546..5633202 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -2071,9 +2071,7 @@
 	size_t clen;
 
 	clen = min_t(const size_t, len, wsize);
-	num_pages = clen / PAGE_CACHE_SIZE;
-	if (clen % PAGE_CACHE_SIZE)
-		num_pages++;
+	num_pages = DIV_ROUND_UP(clen, PAGE_SIZE);
 
 	if (cur_len)
 		*cur_len = clen;