libceph: check for invalid mapping

If we encounter an invalid (e.g., zeroed) mapping, return an error
and avoid a divide by zero.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
diff --git a/include/linux/ceph/osdmap.h b/include/linux/ceph/osdmap.h
index 311ef8d..e88a620 100644
--- a/include/linux/ceph/osdmap.h
+++ b/include/linux/ceph/osdmap.h
@@ -109,9 +109,9 @@
 extern void ceph_osdmap_destroy(struct ceph_osdmap *map);
 
 /* calculate mapping of a file extent to an object */
-extern void ceph_calc_file_object_mapping(struct ceph_file_layout *layout,
-					  u64 off, u64 *plen,
-					  u64 *bno, u64 *oxoff, u64 *oxlen);
+extern int ceph_calc_file_object_mapping(struct ceph_file_layout *layout,
+					 u64 off, u64 *plen,
+					 u64 *bno, u64 *oxoff, u64 *oxlen);
 
 /* calculate mapping of object to a placement group */
 extern int ceph_calc_object_layout(struct ceph_object_layout *ol,