mount: Fix input/output errors for canceled syscalls

[MichaelEischer]

What does this PR change? What problem does it solve?

Accessing restic's fuse mount could result in "input / output" errors for programs in which syscall can be interrupted. This is for example the case for go programs.

bazil/fuse expects us to return context.Canceled to signal that a syscall was successfully interrupted. Returning a wrapped version of that error however causes the fuse library to signal an EIO (input/output error). Thus unwrap context.Canceled errors before returning them.

Was the change previously discussed in an issue or on the forum?

Fixes #3694

Checklist

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• I have added tests for all code changes.
• [ ] I have added documentation for relevant changes (in the manual).
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[MichaelEischer]

Let's merge this. There's only little potential for regressions as the code paths aren't used too often and the basic approach was already tested in #3694.