fusefs: invalidate the cache during copy_file_range

[asomers]

Summary

FUSE_COPY_FILE_RANGE instructs the server to write data to a file.
fusefs must invalidate any cached data within the written range.

Also,
fusefs: inline fuse_io_dispatch

This function was always confusing, because it created an H-shaped
callgraph: two functions called in and left via different paths based on
which which called.

MFC after: 2 weeks

PR: 260242
MFC after: 2 weeks

Test Plan

test case added