Fix Condition.await bug when cancelling

[talex5]

Condition.await needs to re-acquire the lock when cancelled. However, if another fiber is holding the lock then this operation must wait and would itself get cancelled.

Reported by @polytypic.

It's a little annoying that we have to do a non-cancellable wait during cancellation, since typically we only need the lock so we can call unlock on it again immediately. I did try adding a wait_until function that handles all the locking, which allows simplifying this. However, we would need multiple versions of it to cover the various ways of using a lock (use_rw, use_ro, and ~protected), which seems a bit messy.

[polytypic]

The fix seems correct. On quick look the failures don't seem related?

[talex5]

Yes, [WARNING] Failed to allocate 4096 byte fixed buffer means the build machine was a bit overloaded. Normally Eio would fall back to using a regular buffer, but that test is specifically for checking that the fixed buffer works.