fix "iocb" reporting in AsyncIO

Summary: A few fields were swapped, and we had unintentionall fallthrough in
switch.

Reviewed By: luciang

Differential Revision: D2565884

fb-gh-sync-id: 8ad71c090c5120e99c672f785aaefdef03469ee2

folly/experimental/io/AsyncIO.cpp

@@ -348,11 +348,13 @@ std::ostream& operator<<(std::ostream& os, const iocb& cb) {
   switch (cb.aio_lio_opcode) {
     case IO_CMD_PREAD:
     case IO_CMD_PWRITE:
-      os << folly::format("buf={}, off={}, size={}, ",
-                          cb.u.c.buf, cb.u.c.nbytes, cb.u.c.offset);
+      os << folly::format("buf={}, offset={}, nbytes={}, ",
+                          cb.u.c.buf, cb.u.c.offset, cb.u.c.nbytes);
+      break;
     default:
       os << "[TODO: write debug string for "
          << iocbCmdToString(cb.aio_lio_opcode) << "] ";
+      break;
   }
 
   return os;