async: access bottom half flags with qatomic_read

async: access bottom half flags with qatomic_read

Running test-aio-multithread under TSAN reveals data races on bh->flags.
Because bottom halves may be scheduled or canceled asynchronously,
without taking a lock, adjust aio_compute_bh_timeout() and aio_ctx_check()
to use a relaxed read to access the flags.

Use an acquire load to ensure that anything that was written prior to
qemu_bh_schedule() is visible.

Closes: https://gitlab.com/qemu-project/qemu/-/issues/2749
Closes: https://gitlab.com/qemu-project/qemu/-/issues/851
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 5142397c79330aab9bef3230991c8ac0c251110f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>