rwlock: Sprinkle const qualifiers where appropriate

No functional change intended.

MFC after:	2 weeks

sys/kern/kern_rwlock.c

@@ -266,9 +266,9 @@ _rw_destroy(volatile uintptr_t *c)
 }
 
 void
-rw_sysinit(void *arg)
+rw_sysinit(const void *arg)
 {
-	struct rw_args *args;
+	const struct rw_args *args;
 
 	args = arg;
 	rw_init_flags((struct rwlock *)args->ra_rw, args->ra_desc,

sys/sys/rwlock.h

@@ -128,7 +128,7 @@
  */
 void	_rw_init_flags(volatile uintptr_t *c, const char *name, int opts);
 void	_rw_destroy(volatile uintptr_t *c);
-void	rw_sysinit(void *arg);
+void	rw_sysinit(const void *arg);
 int	_rw_wowned(const volatile uintptr_t *c);
 void	_rw_wlock_cookie(volatile uintptr_t *c, const char *file, int line);
 int	__rw_try_wlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF);