s390/stp: Remove udelay from stp_sync_clock()

When an stp sync check is handled on a system with multiple
cpus each cpu gets a machine check but only the first one
actually handles the sync operation. All other CPUs spin
waiting for the first one to finish with a short udelay().
But udelay can't be used here as the first CPU modifies tod_clock_base
before performing the sync op. During this timeframe
get_tod_clock_monotonic() might return a non-monotonic time.

The time spent waiting should be very short and udelay is a busy loop
anyways, therefore simply remove the udelay.

Reviewed-by: Heiko Carstens <[e-mail]>
Signed-off-by: Sven Schnelle <[e-mail]>
Signed-off-by: Alexander Gordeev <[e-mail]>

arch/s390/kernel/time.c

···
 		atomic_dec(&sync->cpus);
 		/* Wait for in_sync to be set. */
 		while (READ_ONCE(sync->in_sync) == 0)
-			__udelay(1);
+			;
 	}
 	if (sync->in_sync != 1)
 		/* Didn't work. Clear per-cpu in sync bit again. */