nfsd: COPY with length 0 should copy to end of file

>From https://tools.ietf.org/html/rfc7862#page-65



	A count of 0 (zero) requests that all bytes from ca_src_offset
	through EOF be copied to the destination.

Reported-by:  <radchenkoy@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>