nfs: fix stub return type warnings

Fix numerous repeated warnings by making the stub function
void instead of non-void:

fs/nfs/nfs4_fs.h: In function 'nfs4_unregister_sysctl':
fs/nfs/nfs4_fs.h:385:1: warning: no return statement in function returning non-void
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc:	Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4_fs.h

@@ -380,7 +380,7 @@ static inline int nfs4_register_sysctl(void)
 	return 0;
 }
 
-static inline int nfs4_unregister_sysctl(void)
+static inline void nfs4_unregister_sysctl(void)
 {
 }
 #endif