Fix incorrect smp::count usage within has_enough_aio_nr()

[fee-mendes]

Within has_enough_aio_nr(), the check has the following logic: aio_max_nr - aio_nr < reactor::max_aio * smp::count which one would naively assume is correct. However, as defined within reactor.cc, smp::count evaluates to 0 during initialization, which means that the function will always skip the conditional and - thus - seastar will fail to fallback to epoll backend.