Remove incorrect mount API and allow null data arg #4057

Job	Run time
Check selected Tier 3 platforms (nightly)	2m 18s
Check (stable)	4m 16s
Check --no-default-features (nightly)	54s
Check nightly-only targets (nightly)	23s
Check (nightly)	4m 6s
Rustfmt	18s
Check (1.63)	7m 32s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 2s
Test use-libc (ubuntu)	38s
Test use-libc (i686-linux)	1m 22s
Test (ubuntu)	42s
Test (ubuntu-20.04)	43s
Test use-libc (aarch64-linux)	1m 18s
Test use-libc (powerpc64le-linux)	1m 55s
Test (i686-linux)	1m 0s
Test (aarch64-linux)	1m 3s
Test use-libc (riscv64-linux)	1m 0s
Test (powerpc64le-linux)	1m 6s
Test use-libc (arm-linux)	1m 3s
Test (riscv64-linux)	1m 12s
Test (s390x-linux)	1m 3s
Test (arm-linux)	1m 34s
Test (ubuntu-stable)	34s
Test (i686-linux-stable)	53s
Test (aarch64-linux-stable)	1m 28s
Test (riscv64-linux-stable)	1m 4s
Test (s390x-linux-stable)	1m 14s
Test (powerpc64le-linux-stable)	1m 5s
Test (arm-linux-stable)	1m 14s
Test (ubuntu-1.63)	2m 59s
Test (i686-linux-1.63)	3m 36s
Test (aarch64-linux-1.63)	3m 24s
Test (riscv64-linux-1.63)	3m 19s
Test (s390x-linux-1.63)	3m 35s
Test (powerpc64le-linux-1.63)	3m 30s
Test (arm-linux-1.63)	3m 24s
Test (macos-latest)	18m 4s
Test (macos-12)	18m 48s
Test (windows)	46s
Test (windows-2019)	49s
Test (musl)	50s
	1h 47m 4s

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