fs/nfsd/nfs4idmap.c: adjust inconsistent IS_ERR and PTR_ERR

Change the call to PTR_ERR to access the value just tested by IS_ERR.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression e,e1;
@@

(
if (IS_ERR(e)) { ... PTR_ERR(e) ... }
|
if (IS_ERR(e=e1)) { ... PTR_ERR(e) ... }
|
*if (IS_ERR(e))
 { ...
*  PTR_ERR(e1)
   ... }
)
// </smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4idmap.c

@@ -478,7 +478,7 @@ nfsd_idmap_init(struct net *net)
 		goto destroy_idtoname_cache;
 	nn->nametoid_cache = cache_create_net(&nametoid_cache_template, net);
 	if (IS_ERR(nn->nametoid_cache)) {
-		rv = PTR_ERR(nn->idtoname_cache);
+		rv = PTR_ERR(nn->nametoid_cache);
 		goto unregister_idtoname_cache;
 	}
 	rv = cache_register_net(nn->nametoid_cache, net);