powerpc/irq: fix stack overflow verification

BugLink: https://bugs.launchpad.net/bugs/1858462

commit 099bc481 upstream.

Before commit 0366a1c7 ("powerpc/irq: Run softirqs off the top of
the irq stack"), check_stack_overflow() was called by do_IRQ(), before
switching to the irq stack.
In that commit, do_IRQ() was renamed __do_irq(), and is now executing
on the irq stack, so check_stack_overflow() has just become almost
useless.

Move check_stack_overflow() call in do_IRQ() to do the check while
still on the current stack.

Fixes: 0366a1c7 ("powerpc/irq: Run softirqs off the top of the irq stack")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e033aa8116ab12b7ca9a9c75189ad0741e3b9b5f.1575872340.git.christophe.leroy@c-s.frSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/powerpc/kernel/irq.c

@@ -484,8 +484,6 @@ void __do_irq(struct pt_regs *regs)
 
 	trace_irq_entry(regs);
 
-	check_stack_overflow();
-
 	/*
 	 * Query the platform PIC for the interrupt & ack it.
 	 *
@@ -517,6 +515,8 @@ void do_IRQ(struct pt_regs *regs)
 	irqtp = hardirq_ctx[raw_smp_processor_id()];
 	sirqtp = softirq_ctx[raw_smp_processor_id()];
 
+	check_stack_overflow();
+
 	/* Already there ? */
 	if (unlikely(curtp == irqtp || curtp == sirqtp)) {
 		__do_irq(regs);