io_uring: do not charge the SQ/CQ rings to RLIMIT_MEMLOCK

Commit 8078486e1d53 ("io_uring: use region api for SQ") and commit
81a4058e0cd0 ("io_uring: use region api for CQ") made io_uring_setup()
allocate the rings with io_create_region().

However, io_create_region() charges the memory to RLIMIT_MEMLOCK, and
the rings had been exempt from that limit since commit 26bfa89e25f4
("io_uring: place ring SQ/CQ arrays under memcg memory limits"). As of
now, a user without CAP_IPC_LOCK gets ENOMEM from io_uring_setup() when
their rings exceed the limit, which is 8 MiB by default. PostgreSQL
developers have already hit this in their io_uring tests [1].

The issue can be reproduced with a simple liburing program, run as an
unprivileged user:

    #include <liburing.h>
    #include <stdio.h>

    int main(void)
    {
            static struct io_uring ring[64];
            int i;

            for (i = 0; i < 64; i++)
                    if (io_uring_queue_init(4096, &ring[i], 0) < 0)
                            break;
            printf("%d rings\n", i);
            return 0;
    }

Before those commits (v6.13), it prints "64 rings". After those commits
(v6.14), it prints "21 rings".

This patch makes io_create_region() take the user to charge, and passes
no user for the SQ/CQ rings.

Link: https://www.postgresql.org/message-id/flat/667f7dcf-404b-4898-a426-57ca6e0617f6@gmail.com [1]
Fixes: 8078486e1d53 ("io_uring: use region api for SQ")
Fixes: 81a4058e0cd0 ("io_uring: use region api for CQ")
Cc: stable@vger.kernel.org
Signed-off-by: Hengyu Liang <hengyul@cs.unc.edu>
Link: https://lore.kernel.org/io-uring/20261006125732.3425762-1-hengyul@cs.unc.edu/
Signed-off-by: Jens Axboe <axboe@kernel.dk>