powerpc/64s/interrupt: Fix lost interrupts when returning to soft-masked context

It's possible for an interrupt returning to an irqs-disabled context to
lose a pending soft-masked irq because it branches to part of the exit
code for irqs-enabled contexts, which is meant to clear only the
PACA_IRQS_HARD_DIS flag from PACAIRQHAPPENED by zeroing the byte. This
just looks like a simple thinko from a recent commit (if there was no
hard mask pending, there would be no reason to clear it anyway).

This also adds comment to the code that actually does need to clear the
flag.

Fixes: e485f6c7 ("powerpc/64/interrupt: Fix return to masked context after hard-mask irq becomes pending")
Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221013064418.1311104-1-npiggin@gmail.com