target-s390x: fix possible out of bounds read

Array index starts at 0, so the valid index of ext_queue array,
io_queue array, mchk_queue array should be MAX_EXT_QUEUE - 1,
MAX_IO_QUEUE - 1, MAX_MCHK_QUEUE - 1.

The original checks missed the invalid bound value, which will lead
possible out of bounds read in the follow codes.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
