Add struct and constants for mount_setattr syscall

[pcc]

Description

Adds struct and constants for mount_setattr syscall.

Sources

https://github.com/torvalds/linux/blob/master/include/uapi/linux/mount.h

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

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[pcc]

@rustbot label stable-nominated

[tgross35]

Ah, you might need to update build.rs if this isn't available on musl

[bors]

☔ The latest upstream changes (presumably #4070) made this pull request unmergeable. Please resolve the merge conflicts.

[tgross35]

@pcc we just updated the linux headers on musl, could you rebase? The tests might start to pass.

[pcc]

@pcc we just updated the linux headers on musl, could you rebase? The tests might start to pass.

Done

[tgross35]

Thanks!