fusefs: fix an interaction between copy_file_range and mmap

[asomers]

Summary

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

MFC after: 2 weeks

Test Plan

two test cases added to the fusefs test suite

[kib]

sys/fs/fuse/fuse_vnops.c

912

Indent is strange.

I suggest to use (newly added) vnode_pager_clear_sync() instead, I do not think that possible optimization of flushing a range is significant there.

[asomers]

• Use vnode_pager_clean_sync