expose AsyncIOOp::getIocb

Summary:
Similarly to `IoUringOp::getSqe()`, expose underlying submission data
structure.  The end goal is to perform certain introspection in client code to
report aggregated stats (number of IO, total size of IO, ...).

Reviewed By: ot

Differential Revision: D28022188

fbshipit-source-id: edc51420ed901f0a0d9b5b1338308ad680371298

folly/experimental/io/AsyncIO.h

@@ -52,6 +52,8 @@ class AsyncIOOp : public AsyncBaseOp {
 
   void toStream(std::ostream& os) const override;
 
+  const iocb& getIocb() const { return iocb_; }
+
  private:
   iocb iocb_;
 };
@@ -76,10 +78,11 @@ class AsyncIO : public AsyncBase {
 
   void initializeContext() override;
 
- private:
+ protected:
   int submitOne(AsyncBase::Op* op) override;
   int submitRange(Range<AsyncBase::Op**> ops) override;
 
+ private:
   Range<AsyncBase::Op**> doWait(
       WaitType type,
       size_t minRequests,

folly/experimental/io/IoUring.h

@@ -98,10 +98,11 @@ class IoUring : public AsyncBase {
 
   void initializeContext() override;
 
- private:
+ protected:
   int submitOne(AsyncBase::Op* op) override;
   int submitRange(Range<AsyncBase::Op**> ops) override;
 
+ private:
   Range<AsyncBase::Op**> doWait(
       WaitType type,
       size_t minRequests,