Changes

Remove old empty io_uring

A previous patch creates a new io_uring, if it becomes too small to
store all SQEs. When all SQEs of the old ring are completed, the old
ring will be destroyed.

A counter is incremented whenever an SQE is submitted to an io_uring.
The counter is decremented whenever a CQE is received and handled. This
counter will determine when a ring is empty and can be destroyed.

Related: OS#6705
Change-Id: Id2d2a0400ad442198c684ea0ead4eaeaead4c53d