cifs: set correct max-buffer-size for smb2_ioctl_init()

Fix two places where we need to adjust down the max response size for
ioctl when it is used together with compounding.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
CC: Stable <stable@vger.kernel.org>