```
This ensures that *mpp != NULL iff vn_finished_write() should be called,
regardless of the returned error. The only exception that must be
maintained is the case where vn_start_write(V_NOWAIT) is called with the
intent of later dropping other locks and then doing
vn_start_write(V_XSLEEP), which needs the mp value calculated from the
non-waitable call above it. Add V_FORXSLEEP flag, which could be
combined with V_NOWAIT, and use it in such situations.
Also note that V_XSLEEP is not supported by vn_start_secondary_write().
```