fix: Type-convert fs.Bavail for portability

[ayang64]

Prior to this patch, DiskUsage() would calculate bytes available
by multiplying blocks available by block size in bytes:

disk.Avail = fs.Bavail * uint64(fs.Bsize)

Under some versions of Unix, fs.Bavail is of type uint64 and on
others (like FreeBSD) it is of type int64.

This causes a compile time error:

  $ go build
  # github.com/influxdata/influxdb/v2/pkg/fs
  ./fs_unix.go:81:25: invalid operation: fs.Bavail * uint64(fs.Bsize) (mismatched types int64 and uint64)

This patch type-converts fs.Bavail to unit64 to ensure that all
types in the expression align.

This prevents compile time errors under FreeBSD and other platforms
where fs.Bavail isn't uint64.

Closes #

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• CHANGELOG.md updated with a link to the PR (not the Issue)
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[vlastahajek]

Thanks for the fix!