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>

fs/nfsd/nfs4proc.c

@@ -1387,6 +1387,9 @@ static ssize_t _nfsd_copy_file_range(struct nfsd4_copy *copy)
 	u64 src_pos = copy->cp_src_pos;
 	u64 dst_pos = copy->cp_dst_pos;
 
+	/* See RFC 7862 p.67: */
+	if (bytes_total == 0)
+		bytes_total = ULLONG_MAX;
 	do {
 		if (kthread_should_stop())
 			break;