Mmap flush does not require mutable reference, use corret mut ref self

[timvisee]

This does three things:

• Flushing does not require a mutable reference, because the kernel uses a global mutex. So we switched from inner_mut() to inner().
• Use &mut self rather than &self in inner_mut() method. Remove clippy error bypass for it.
• Only implement Sync on MmapViewSync for testing.

I believe more may be wrong with this regarding soundness. But possibly is a good start for some improvements.

[agourlay]

Not entirely sure but this looks legit 👍

[timvisee]

Not entirely sure but this looks legit 👍

The above is also the reason why flush() got a &self rather than a &mut self.