libceph: fix broken data length assertions

It's OK for the result of a read to come back with fewer bytes than
were requested.  So don't trigger a BUG() in that case when
initializing the data cursor.

This resolves the first problem described in:
    http://tracker.ceph.com/issues/4598Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>

net/ceph/messenger.c

@@ -833,7 +833,7 @@ static void ceph_msg_data_pages_cursor_init(struct ceph_msg_data *data,
 
 	BUG_ON(!data->pages);
 	BUG_ON(!data->length);
-	BUG_ON(length != data->length);
+	BUG_ON(length > data->length);	/* short reads are OK */
 
 	cursor->resid = length;
 	page_count = calc_pages_for(data->alignment, (u64)data->length);
@@ -905,7 +905,7 @@ static void ceph_msg_data_pagelist_cursor_init(struct ceph_msg_data *data,
 
 	pagelist = data->pagelist;
 	BUG_ON(!pagelist);
-	BUG_ON(length != pagelist->length);
+	BUG_ON(length > pagelist->length);	/* short reads are OK */
 
 	if (!length)
 		return;		/* pagelist can be assigned but empty */