NFS: Store the legacy idmapper result in the keyring

This patch removes the old hashmap-based caching and instead uses a
"request key actor" to place an upcall to the legacy idmapper rather
than going through /sbin/request-key.  This will only be used as a
fallback if /etc/request-key.conf isn't configured to use nfsidmap.
Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/nfs_idmap.h

@@ -69,19 +69,8 @@ struct nfs_server;
 struct nfs_fattr;
 struct nfs4_string;
 
-#ifdef CONFIG_NFS_V4
 int nfs_idmap_init(void);
 void nfs_idmap_quit(void);
-#else
-static inline int nfs_idmap_init(void)
-{
-	return 0;
-}
-
-static inline void nfs_idmap_quit(void)
-{}
-#endif
-
 int nfs_idmap_new(struct nfs_client *);
 void nfs_idmap_delete(struct nfs_client *);