Fix procfs on /proc/self with user namespaces #3325

Job	Run time
Check nightly-only targets (nightly)	36s
Check (stable)	10m 23s
Check selected Tier 3 platforms (nightly)	5m 4s
Check --no-default-features (nightly)	23s
Check (nightly)	9m 38s
Check (1.63)	15m 29s
Rustfmt	11s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 28s
Test (ubuntu)	1m 21s
Test use-libc (ubuntu)	1m 5s
Test (ubuntu-20.04)	1m 22s
Test use-libc (i686-linux)	1m 35s
Test (i686-linux)	1m 52s
Test use-libc (aarch64-linux)	1m 35s
Test (aarch64-linux)	2m 10s
Test use-libc (powerpc64le-linux)	2m 11s
Test (powerpc64le-linux)	1m 58s
Test use-libc (riscv64-linux)	1m 55s
Test (riscv64-linux)	1m 28s
Test use-libc (arm-linux)	1m 44s
Test (s390x-linux)	1m 58s
Test (arm-linux)	1m 31s
Test (ubuntu-stable)	1m 18s
Test (i686-linux-stable)	1m 23s
Test (aarch64-linux-stable)	1m 34s
Test (riscv64-linux-stable)	1m 49s
Test (s390x-linux-stable)	1m 30s
Test (powerpc64le-linux-stable)	1m 39s
Test (arm-linux-stable)	1m 52s
Test (ubuntu-1.63)	4m 17s
Test (i686-linux-1.63)	4m 58s
Test (aarch64-linux-1.63)	5m 47s
Test (riscv64-linux-1.63)	4m 37s
Test (s390x-linux-1.63)	6m 10s
Test (powerpc64le-linux-1.63)	4m 54s
Test (arm-linux-1.63)	4m 51s
Test (macos-latest)	1m 23s
Test (macos-11)	1m 10s
Test (windows)	1m 3s
Test (windows-2019)	1m 21s
	1h 58m 33s

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