powerpc/watchdog: Do not panic from locked CPU's IPI handler

The SMP watchdog will detect locked CPUs and IPI them to print a
backtrace and registers. If panic on hard lockup is enabled, do not
panic from this handler, because that can cause recursion into the IPI
layer during the panic.

The caller already panics in this case.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>