Make the NFSv4.2 server fail Copy when from and to files are the same.

The Linux copy_file_range() now allows the in_fd and out_fd to refer to
the same file. The Linux client implements this case using read/write
instead of the Copy operation, which is correct, since RFC-7862 does not
allow Copy to be done with both file handles referring to the same file.
When I looked, the server did not check for this case, so this patch adds
code to fail the Copy when both in and out file handles refer to the same
file.