cifs: fix pcchunk length type in smb2_copychunk_range

Change type of pcchunk->Length from u32 to u64 to match
smb2_copychunk_range arguments type. Fixes the problem where performing
server-side copy with CIFS_IOC_COPYCHUNK_FILE ioctl resulted in incomplete
copy of large files while returning -EINVAL.

Fixes: 9bf0c9cd ("CIFS: Fix SMB2/SMB3 Copy offload support (refcopy) for large files")
Cc: <stable@vger.kernel.org>
Signed-off-by: Pawel Witek <pawel.ireneusz.witek@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>