libceph: record byte count not page count

Record the byte count for an osd request rather than the page count.
The number of pages can always be derived from the byte count (and
alignment/offset) but the reverse is not true.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
diff --git a/include/linux/ceph/osd_client.h b/include/linux/ceph/osd_client.h
index 40e0260..a8016df 100644
--- a/include/linux/ceph/osd_client.h
+++ b/include/linux/ceph/osd_client.h
@@ -63,7 +63,7 @@
 	union {
 		struct {
 			struct page	**pages;
-			u32		num_pages;
+			u64		length;
 			u32		alignment;
 			bool		pages_from_pool;
 			bool		own_pages;