aio: make aio_context_acquire()/aio_context_release() a no-op

aio_context_acquire()/aio_context_release() has been replaced by
fine-grained locking to protect state shared by multiple threads. The
AioContext lock still plays the role of balancing locking in
AIO_WAIT_WHILE() and many functions in QEMU either require that the
AioContext lock is held or not held for this reason. In other words, the
AioContext lock is purely there for consistency with itself and serves
no real purpose anymore.

Stop actually acquiring/releasing the lock in
aio_context_acquire()/aio_context_release() so that subsequent patches
can remove callers across the codebase incrementally.

I have performed "make check" and qemu-iotests stress tests across
x86-64, ppc64le, and aarch64 to confirm that there are no failures as a
result of eliminating the lock.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20231205182011.1976568-5-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

util/async.c

@@ -725,12 +725,12 @@ void aio_context_unref(AioContext *ctx)
 
 void aio_context_acquire(AioContext *ctx)
 {
-    qemu_rec_mutex_lock(&ctx->lock);
+    /* TODO remove this function */
 }
 
 void aio_context_release(AioContext *ctx)
 {
-    qemu_rec_mutex_unlock(&ctx->lock);
+    /* TODO remove this function */
 }
 
 QEMU_DEFINE_STATIC_CO_TLS(AioContext *, my_aiocontext)