fuse: fix copy_file_range cache issues

a) Dirty cache needs to be written back not just in the writeback_cache
case, since the dirty pages may come from memory maps.

b) The fuse_writeback_range() helper takes an inclusive interval, so the
end position needs to be pos+len-1 instead of pos+len.

Fixes: 88bc7d50 ("fuse: add support for copy_file_range()")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>