powerpc/64s: Use dec_mm_active_cpus helper

Avoid open-coded atomic_dec on mm->context.active_cpus and use the
function made for it. Add CONFIG_DEBUG_VM underflow checking on the
counter.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230524060821.148015-3-npiggin@gmail.com