grow: Fix size source

ext_scripts/ext_rbd.py

@@ -238,6 +238,7 @@ def read_env():
 
     env = {"name": os.getenv("VOL_NAME"),
            "size": os.getenv("VOL_SIZE"),
+           "new_size": os.getenv("VOL_NEW_SIZE"),
            "snapshot_name": os.getenv("VOL_SNAPSHOT_NAME"),
            "cephx": cephx,
            "reuse_data": reuse_data,
@@ -383,7 +384,7 @@ def detach(env):
 def grow(env):
     """Grow an existing RBD Image"""
     name = env.get("name")
-    size = env.get("size")
+    size = env.get("new_size")
     pool = env.get("rbd_pool")
     cephx = env.get("cephx")