powerpc/64/interrupt: Fix false warning in context tracking due to idle state

Commit 17147677 ("context_tracking: Convert state to atomic_t")
added a CONTEXT_IDLE state which can be encountered by interrupts from
kernel mode in the idle thread, causing a false positive warning.

Fixes: 17147677

 ("context_tracking: Convert state to atomic_t")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220926054305.2671436-2-npiggin@gmail.com