ARC: remove redundant READ_ONCE() in cmpxchg loop

This patch reverts commit 7082a29c ("ARC: use ACCESS_ONCE in cmpxchg
loop").

It is not necessary to use READ_ONCE() because cmpxchg contains barrier. We
can get it from commit d57f7272 ("ARC: add compiler barrier to LLSC
based cmpxchg").
Signed-off-by: Bang Li <libang.linuxer@gmail.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>

arch/arc/kernel/smp.c

@@ -274,7 +274,7 @@ static void ipi_send_msg_one(int cpu, enum ipi_msg_type msg)
 	 * and read back old value
 	 */
 	do {
-		new = old = READ_ONCE(*ipi_data_ptr);
+		new = old = *ipi_data_ptr;
 		new |= 1U << msg;
 	} while (cmpxchg(ipi_data_ptr, old, new) != old);