mount: Make use of MNT_DOOMED #1119

Job	Run time
build-minimal-sysroots (x86_64)	49s
build-minimal-sysroots (riscv64)	55s
build-minimal-sysroots (arm64)	1m 2s
build-toolchains (ubuntu-latest, x86_64, gnu)	36s
build-toolchains (ubuntu-latest, riscv64, gnu)	36s
build-toolchains (ubuntu-latest, arm64, gnu)	36s
build-toolchains (ubuntu-20.04, x86_64, gnu)	44s
build-toolchains (ubuntu-20.04, riscv64, gnu)	40s
build-toolchains (ubuntu-20.04, arm64, gnu)	27s
build-toolchains (macos-latest, x86_64, gnu)	45s
build-toolchains (macos-latest, riscv64, gnu)	56s
build-toolchains (macos-latest, arm64, gnu)	43s
build-toolchains (ubuntu-latest, llvm, all)	1m 5s
build	3m 16s
build-llvm	3m 16s
build-riscv	2m 42s
build-llvm-riscv64	2m 56s
build-arm64	2m 37s
build-llvm-arm64	2m 48s
run-riscv	0s
run-x86_64	0s
run-x86_64-llvm	0s
run-riscv64-llvm	0s
	27m 29s

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