CIFS: Fix SMB2/SMB3 Copy offload support (refcopy) for large files

This third version of the patch, incorparating feedback from David Disseldorp
extends the ability of copychunk (refcopy) over smb2/smb3 mounts to
handle servers with smaller than usual maximum chunk sizes
and also fixes it to handle files bigger than the maximum chunk sizes

In the future this can be extended further to handle sending
multiple chunk requests in on SMB2 ioctl request which will
further improve performance, but even with one 1MB chunk per
request the speedup on cp is quite large.

Reviewed-by: David Disseldorp <ddiss@samba.org>
Signed-off-by: Steve French <smfrench@gmail.com>