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 da7226993f.

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

src/backend/storage/aio/aio_init.c

@@ -30,12 +30,8 @@
 static Size
 AioCtlShmemSize(void)
 {
-	Size		sz;
-
 	/* pgaio_ctl itself */
-	sz = offsetof(PgAioCtl, io_handles);
-
-	return sz;
+	return sizeof(PgAioCtl);
 }
 
 static uint32