powerpc/mce: fix off by one errors in mce event handling

Before 69111bac42f5 ("powerpc: Replace __get_cpu_var uses"), in
save_mce_event, index got the value of mce_nest_count, and
mce_nest_count was incremented *after* index was set.

However, that patch changed the behaviour so that mce_nest count was
incremented *before* setting index.

This causes an off-by-one error, as get_mce_event sets index as
mce_nest_count - 1 before reading mce_event. Thus get_mce_event reads
bogus data, causing warnings like
"Machine Check Exception, Unknown event version 0 !"
and breaking MCEs handling.

Restore the old behaviour and unbreak MCE handling by subtracting one
from the newly incremented value.

The same broken change occured in machine_check_queue_event (which set
a queue read by machine_check_process_queued_event). Fix that too,
unbreaking printing of MCE information.

Fixes: 69111bac42f5 ("powerpc: Replace __get_cpu_var uses")
CC: [e-mail]
CC: Mahesh Salgaonkar <[e-mail]>
CC: Christoph Lameter <[e-mail]>
Signed-off-by: Daniel Axtens <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/kernel/mce.c

···
 		    uint64_t nip, uint64_t addr)
 {
 	uint64_t srr1;
-	int index = __this_cpu_inc_return(mce_nest_count);
+	int index = __this_cpu_inc_return(mce_nest_count) - 1;
 	struct machine_check_event *mce = this_cpu_ptr(&mce_event[index]);
 
 	/*
···
 	if (!get_mce_event(&evt, MCE_EVENT_RELEASE))
 		return;
 
-	index = __this_cpu_inc_return(mce_queue_count);
+	index = __this_cpu_inc_return(mce_queue_count) - 1;
 	/* If queue is full, just return for now. */
 	if (index >= MAX_MC_EVT) {
 		__this_cpu_dec(mce_queue_count);