fio: Use atomic_load_acquire() and atomic_store_release() where appropriate

fio: Use atomic_load_acquire() and atomic_store_release() where appropriate

The write_barrier() in io_completed() and also the read barriers in
verify.c are misplaced: the write barrier should occur before the flags
update instead of after and the read barriers should occur after the flags
read instead of before. Fix this by using atomic_{load_acquire,
store_release}() instead of read and write barriers.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>