MDEV-25760: Resubmit IO job on -EAGAIN from io_uring

The server still may abort if there is no enough free space in the
ring buffer to resubmit the IO job, but the behavior is equal to
the failure of os_aio() -> submit_io().

tpool/aio_liburing.cc

@@ -162,6 +162,13 @@ class aio_uring final : public tpool::aio
 
       io_uring_cqe_seen(&aio->uring_, cqe);
 
+      if (res == -EAGAIN) {
+        // If we need to resubmit the IO operation, but the ring is full,
+        // then just go the same path as for any other error codes.
+        if (!aio->submit_io(iocb))
+          continue;
+      }
+
       iocb->m_internal_task.m_func= iocb->m_callback;
       iocb->m_internal_task.m_arg= iocb;
       iocb->m_internal_task.m_group= iocb->m_group;