Fix copy-pasto with process count calculation in method_io_uring.c

Fix copy-pasto with process count calculation in method_io_uring.c

This commit replaces the formula used for "TotalProcs" with a call to
pgaio_uring_procs() in pgaio_uring_shmem_init() for the shared memory
initialization, which is exactly the same, removing a duplication.

pgaio_uring_procs() is used for shared memory sizing and a sanity check,
and it has some documentation explaining some reasoning behind the
formula.

Author: Japin Li <japinli@hotmail.com>
Discussion: https://postgr.es/m/ME0P300MB044521067A1EDDA9EDEC3793B66DA@ME0P300MB0445.AUSP300.PROD.OUTLOOK.COM