Fix shared memory calculation size of PgAioCtl

Fix shared memory calculation size of PgAioCtl

The shared memory size was calculated based on an offset of io_handles,
which is itself a pointer included in the structure.  We tend to
overestimate the shared memory size overall, so this was unlikely an
issue in practice, but let's be correct and use the full size of the
structure in the calculation, so as the pointer for io_handles is
included.

Oversight in da7226993fd4.

Author: Madhukar Prasad <madhukarprasad@google.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/CAKi+wrbC2dTzh_vKJoAZXV5wqTbhY0n4wRNpCjJ=e36aoo0kFw@mail.gmail.com
Backpatch-through: 18