hrtimer: don't modify restart_block->fn in restart functions

hrtimer_nanosleep_restart() clears/restores restart_block->fn. This is
pointless and complicates its usage. Note that if sys_restart_syscall()
doesn't actually happen, we have a bogus "pending" restart->fn anyway,
this is harmless.

Signed-off-by: Oleg Nesterov <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Cc: Pavel Emelyanov <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Toyo Abe <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>

kernel/compat.c

···
 	set_fs(oldfs);
 
 	if (ret) {
-		restart->fn = compat_nanosleep_restart;
 		restart->arg1 = (unsigned long)rmtp;
 
 		if (rmtp && put_compat_timespec(&rmt, rmtp))

kernel/hrtimer.c

···
 	struct hrtimer_sleeper t;
 	struct timespec __user  *rmtp;
 
-	restart->fn = do_no_restart_syscall;
-
 	hrtimer_init(&t.timer, restart->arg0, HRTIMER_MODE_ABS);
 	t.timer.expires.tv64 = ((u64)restart->arg3 << 32) | (u64) restart->arg2;
 
···
 		if (ret <= 0)
 			return ret;
 	}
-
-	restart->fn = hrtimer_nanosleep_restart;
 
 	/* The other values in restart are already filled in */
 	return -ERESTART_RESTARTBLOCK;