Fix procfs on /proc/self with user namespaces #3330

Job	Run time
Rustfmt	7s
Check --no-default-features (nightly)	1m 25s
Check nightly-only targets (nightly)	43s
Check (stable)	8m 8s
Check selected Tier 3 platforms (nightly)	4m 40s
Check (nightly)	8m 20s
Check (1.63)	14m 52s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 37s
Test use-libc (ubuntu)	57s
Test use-libc (i686-linux)	2m 6s
Test (ubuntu)	1m 3s
Test use-libc (aarch64-linux)	2m 24s
Test (ubuntu-20.04)	1m 13s
Test use-libc (powerpc64le-linux)	1m 33s
Test (i686-linux)	1m 29s
Test use-libc (riscv64-linux)	1m 27s
Test (aarch64-linux)	1m 36s
Test use-libc (arm-linux)	2m 1s
Test (powerpc64le-linux)	1m 37s
Test (riscv64-linux)	1m 37s
Test (s390x-linux)	1m 51s
Test (arm-linux)	1m 44s
Test (ubuntu-stable)	1m 5s
Test (i686-linux-stable)	1m 28s
Test (aarch64-linux-stable)	1m 23s
Test (riscv64-linux-stable)	1m 28s
Test (s390x-linux-stable)	1m 51s
Test (powerpc64le-linux-stable)	2m 34s
Test (arm-linux-stable)	2m 8s
Test (ubuntu-1.63)	4m 18s
Test (i686-linux-1.63)	4m 42s
Test (aarch64-linux-1.63)	5m 16s
Test (riscv64-linux-1.63)	4m 51s
Test (s390x-linux-1.63)	5m 31s
Test (powerpc64le-linux-1.63)	6m 24s
Test (arm-linux-1.63)	5m 12s
Test (macos-latest)	1m 20s
Test (macos-11)	1m 16s
Test (windows)	1m 15s
Test (windows-2019)	1m 20s
	1h 55m 52s

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