NetBSD: fix getmntinfo for NetBSD

[0-wiz-0]

Description

This patch fixes libc::getmntinfo on NetBSD.

Sources

The problem is that NetBSD provides binary compatibility in a way that was not considered when initially providing the implementation.
Once a symbol name has been used, it stays the same, so the original getmntinfo() in NetBSD, using struct statfs, will always be available as getmntinfo. When getmntinfo was changed to provide struct statvfs (note the 'v') a long time ago, a new symbol was introduced, __getmntinfo13 and the system headers rename access to getmntinfo to call __getmntinfo13 instead.
The rust implementation called the old symbol (providing struct statfs) but parsed the returned information as struct statvfs, leading to wrong data and segfaults.

Checklist

The change has been tested on NetBSD 9, 10, and -current (all supported versions).
No compatibility for NetBSD 8 or older is required, since these versions are not supported any longer; the symbol rename happened before NetBSD 9.

[rustbot]

r? @tgross35

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[0-wiz-0]

This should be pulled up, so
@rustbot label stable-nominated

[tgross35]

Makes sense to me. Our statvfs is actually statvfs90 then? (Based on the call here https://github.com/NetBSD/src/blob/ea69897761f4bc39da7326529c06cc55f32a6d1c/lib/libc/compat/gen/compat___getmntinfo13.c#L52-L58)

[0-wiz-0]

Exactly. We could switch to the new statvfs with the one additional field, but it's a bigger change and we'd need to differentiate between NetBSD 9 and 10+, so i'd leave that for another time.