arm64: alternatives: ensure secondary CPUs execute ISB after patching

In order to guarantee that the patched instruction stream is visible to
a CPU, that CPU must execute an isb instruction after any related cache
maintenance has completed.

The instruction patching routines in kernel/insn.c get this right for
things like jump labels and ftrace, but the alternatives patching omits
it entirely leaving secondary cores in a potential limbo between the old
and the new code.

This patch adds an isb following the secondary polling loop in the
altenatives patching.
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/alternative.c

@@ -132,6 +132,7 @@ static int __apply_alternatives_multi_stop(void *unused)
 	if (smp_processor_id()) {
 		while (!READ_ONCE(patched))
 			cpu_relax();
+		isb();
 	} else {
 		BUG_ON(patched);
 		__apply_alternatives(&region);