nfs: fix mispelling of idmap CONFIG symbol

Trivial, but confusing when you're trying to grep through this
code....
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/idmap.c

@@ -238,7 +238,7 @@ int nfs_map_gid_to_group(struct nfs_client *clp, __u32 gid, char *buf, size_t bu
 	return nfs_idmap_lookup_name(gid, "group", buf, buflen);
 }
 
-#else  /* CONFIG_NFS_USE_IDMAPPER not defined */
+#else  /* CONFIG_NFS_USE_NEW_IDMAPPER not defined */
 
 #include <linux/module.h>
 #include <linux/mutex.h>