io_uring engine: 'atomic_load_relaxed' instead of 'atomic_load_acquire'

io_uring engine: 'atomic_load_relaxed' instead of 'atomic_load_acquire'

motivation here is that we do not have here any explicit READ dependency
 on atomic load because actually we just need in these places only
 operation to perform atomically without any explicit barriers given by
 memory model

Signed-off-by: Denis Pronin <dannftk@yandex.ru>