cifs: fix potential overflow in cifs_compose_mount_options

In worst case, "ip=" + sb_mountdata + ipv6 can be copied into mountdata.
Therefore, for safe, it is better to add more size when allocating memory.

Signed-off-by: Insu Yun <wuninsu@gmail.com>
Signed-off-by: Steve French <smfrench@gmail.com>