NFS: Create a free_client rpc_op

NFS v4 needs a way to shut down callbacks and sessions, but v2 and v3
don't.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/nfs/proc.c b/fs/nfs/proc.c
index 1663293..53620bf 100644
--- a/fs/nfs/proc.c
+++ b/fs/nfs/proc.c
@@ -791,4 +791,5 @@
 	.have_delegation = nfs_have_delegation,
 	.return_delegation = nfs_return_delegation,
 	.init_client	= nfs_init_client,
+	.free_client	= nfs_free_client,
 };