os/bluestore/BlueStore: for read, length=0 mean length=object.size.

Like FileStore, if length=0 mean length=object.size.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>

src/os/bluestore/BlueStore.cc

@@ -2522,7 +2522,7 @@ int BlueStore::read(
     goto out;
   }
 
-  if (offset == length && offset == 0)
+  if (length == 0)
     length = o->onode.size;
 
   r = _do_read(o, offset, length, bl, op_flags);