fusefs: fix an interaction between copy_file_range and mmap

If a copy_file_range operation tries to read from a page that was
previously written via mmap, that page must be flushed first.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D43451

(cherry picked from commit 1c909c300b92601f7690610097ac98126caff835)