osdmap: successfully decode short map

When we send (old) maps to the kclient, we omit the extended section.  Lets
decode those (old, abbreviated maps) successfully, too.

Signed-off-by: Sage Weil <sage@newdream.net>

src/osd/OSDMap.cc

@@ -243,6 +243,10 @@ void OSDMap::Incremental::decode(bufferlist::iterator &p)
     ::decode(new_pg_temp, p);
   }
 
+  // decode short map, too.
+  if (v == 5 && p.end())
+    return;
+
   // extended
   __u16 ev = 0;
   if (v >= 5)