nfs,rcu: convert call_rcu(nfs_free_delegation_callback) to kfree_rcu()

The rcu callback nfs_free_delegation_callback() just calls a kfree(),
so we use kfree_rcu() instead of the call_rcu(nfs_free_delegation_callback).

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>