powerpc/64: Fix an out of date comment about MMIO ordering

This primitive has been renamed, but because it was spelled incorrectly in the
first place it must have escaped the fixup patch.  As far as I can tell this
logic is still correct: smp_mb__after_spinlock() uses the default smp_mb()
implementation, which is "sync" rather than "hwsync" but those are the same
(though I'm not that familiar with PowerPC).
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200716193820.1141936-1-palmer@dabbelt.com

arch/powerpc/kernel/entry_64.S

@@ -354,7 +354,7 @@ _GLOBAL(_switch)
 	 * kernel/sched/core.c).
 	 *
 	 * Uncacheable stores in the case of involuntary preemption must
-	 * be taken care of. The smp_mb__before_spin_lock() in __schedule()
+	 * be taken care of. The smp_mb__after_spinlock() in __schedule()
 	 * is implemented as hwsync on powerpc, which orders MMIO too. So
 	 * long as there is an hwsync in the context switch path, it will
 	 * be executed on the source CPU after the task has performed