pgsql: aio: Combine io_uring memory mappings, if supported

aio: Combine io_uring memory mappings, if supported

By default io_uring creates a shared memory mapping for each io_uring
instance, leading to a large number of memory mappings. Unfortunately a large
number of memory mappings slows things down, backend exit is particularly
affected. To address that, newer kernels (6.5) support using user-provided
memory for the memory. By putting the relevant memory into shared memory we
don't need any additional mappings.

On a system with a new enough kernel and liburing, there is no discernible
overhead when doing a pgbench -S -C anymore.

Reported-by: MARK CALLAGHAN <mdcallag(at)gmail(dot)com>
Reviewed-by: "Burd, Greg" <greg(at)burd(dot)me>
Reviewed-by: Jim Nasby <jnasby(at)upgrade(dot)com>
Discussion: https://postgr.es/m/CAFbpF8OA44_UG+RYJcWH9WjF7E3GA6gka3gvH6nsrSnEe9H0NA@mail.gmail.com
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9a5334c0b4e9d7269ee9b5e9d08925c8a1e1e01e

Modified Files
--------------
configure | 17 +++
configure.ac | 7 +
meson.build | 6 +
src/backend/storage/aio/method_io_uring.c | 210 +++++++++++++++++++++++++++++-
src/include/pg_config.h.in | 3 +
src/tools/pgindent/typedefs.list | 1 +
6 files changed, 238 insertions(+), 6 deletions(-)