nfs41: implement DESTROY_CLIENTID operation

According to rfc5661 18.50, implement DESTROY_CLIENTID operation.

Signed-off-by: Mi Jinlong <mijinlong@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index 458ebb6..fa38336 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -1645,7 +1645,7 @@
 		.op_name = "OP_SEQUENCE",
 	},
 	[OP_DESTROY_CLIENTID] = {
-		.op_func = NULL,
+		.op_func = (nfsd4op_func)nfsd4_destroy_clientid,
 		.op_flags = ALLOWED_WITHOUT_FH | ALLOWED_AS_FIRST_OP
 				| OP_MODIFIES_SOMETHING,
 		.op_name = "OP_DESTROY_CLIENTID",