copy_file_range return value on FUSE

The FUSE protocol uses struct fuse_write_out to convey the return value
of copy_file_range, which is restricted to uint32_t.  But the
copy_file_range interface supports a 64-bit copy operation.  Given that
copy_file_range is expected to clone huge files, large copies are not
unexpected, so this appears to be a real limitation.

There is another wrinkle: we'd need to check if the process runs in
32-bit compat mode, and reject size_t arguments larger than INT_MAX in
this case (with EOVERFLOW presumably).  But perhaps this should be
handled on the kernel side?  Currently, this doesn't seem to happen, and
we can get copy_file_range results in the in-band error range.
Applications have no way to disambiguate this.

Thanks,
Florian