powerpc/syscall: switch user_exit_irqoff and trace_hardirqs_off order

user_exit_irqoff() -> __context_tracking_exit -> vtime_user_exit
warns in __seqprop_assert due to lockdep thinking preemption is enabled
because trace_hardirqs_off() has not yet been called.

Switch the order of these two calls, which matches their ordering in
interrupt_enter_prepare.

Fixes: 5f0b6ac3 ("powerpc/64/syscall: Reconcile interrupts")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210316104206.407354-2-npiggin@gmail.com