fix pg_stat_get_backend_subxact() to use the right backend ID

When Who What
2023-08-30 21:56:02 Nathan Bossart (bossartn) Closed in commitfest 2023-09 with status: Committed
2023-08-30 21:56:02 Nathan Bossart (bossartn) Changed committer to bossartn
2023-08-30 02:02:42 Nathan Bossart (bossartn) New status: Needs review
2023-08-28 05:29:49 Michael Paquier (michael-kun) New status: Waiting on Author
2023-08-24 16:20:25 Nathan Bossart (bossartn) Changed authors to Ian Barwick (barwick), Nathan Bossart (bossartn)
2023-08-24 16:20:25 Nathan Bossart (bossartn) Changed targetversion to 16
2023-08-24 16:20:13 Nathan Bossart (bossartn) Attached mail thread CAB8KJ=j-ACb3H4L9a_b3ZG3iCYDW5aEu3WsPAzkm2S7JzS1Few@mail.gmail.com
2023-08-24 16:20:13 Nathan Bossart (bossartn) Created patch record