Fix AsyncIO reading seed queueing (facebook#3940)

Fixes a bug in AsyncIO where we queue reads after opening a file so our queue will always be saturated (or as saturated as possible).
Previous code was looping up to `availableJobsCount` not realizing `availableJobsCount` was also decreasing in each iteration, so instead of queueing 10 jobs we'd queue 5 (and instead of 2 we'd queue 1).
This PR fixes the loop to queue as long as `availableJobsCount` is not 0.

Author: yoniko

programs/fileio_asyncio.c

@@ -514,8 +514,7 @@ static void AIO_ReadPool_enqueueRead(ReadPoolCtx_t* ctx) {
 }
 
 static void AIO_ReadPool_startReading(ReadPoolCtx_t* ctx) {
-    int i;
-    for (i = 0; i < ctx->base.availableJobsCount; i++) {
+    while(ctx->base.availableJobsCount) {
         AIO_ReadPool_enqueueRead(ctx);
     }
 }