Add `#[non_exhaustive]` to io_uring structs with padding #4624

Job	Run time
Check nightly-only targets (nightly)	23s
Check selected Tier 3 platforms (nightly)	4m 48s
Check --no-default-features (nightly)	56s
Check (stable)	6m 9s
Rustfmt	11s
Check (nightly)	5m 57s
Check (1.63)	9m 57s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 19s
Test rustix_use_experimental_asm (s390x-linux)	1m 16s
Test use-libc (ubuntu)	43s
Test (ubuntu)	43s
Test use-libc (i686-linux)	1m 19s
Test (ubuntu-20.04)	44s
Test use-libc (aarch64-linux)	1m 29s
Test (i686-linux)	1m 12s
Test use-libc (powerpc64le-linux)	1m 13s
Test use-libc (riscv64-linux)	2m 8s
Test use-libc (s390x-linux)	1m 19s
Test (aarch64-linux)	1m 11s
Test use-libc (arm-linux)	2m 19s
Test (powerpc64le-linux)	1m 12s
Test (riscv64-linux)	1m 23s
Test (s390x-linux)	1m 16s
Test (arm-linux)	2m 59s
Test (ubuntu-stable)	48s
Test (i686-linux-stable)	1m 18s
Test (aarch64-linux-stable)	1m 21s
Test (riscv64-linux-stable)	1m 7s
Test (s390x-linux-stable)	1m 10s
Test (powerpc64le-linux-stable)	1m 25s
Test (arm-linux-stable)	1m 15s
Test (ubuntu-1.63)	2m 26s
Test (i686-linux-1.63)	2m 51s
Test (aarch64-linux-1.63)	3m 4s
Test (riscv64-linux-1.63)	3m 14s
Test (s390x-linux-1.63)	2m 52s
Test (powerpc64le-linux-1.63)	3m 5s
Test (arm-linux-1.63)	2m 53s
Test (macos-latest)	30s
Test (macos-13)	1m 0s
Test (windows)	49s
Test (windows-2019)	1m 10s
Test (musl)	45s
	1h 25m 9s

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