target: accept REQUEST_SENSE with 18bytes

WindowsXP+BOT issues a MODE_SENSE request with page 0x1c which is not
suppoerted by target. Target rejects that command with
TCM_INVALID_CDB_FIELD, so far so good. On BOT I can't send the SENSE
response back, instead I can only reply that an error occured. The next
thing happens is a REQUEST_SENSE request with 18 bytes length. Since the
check here is more than 18 bytes I have to NACK that request as well.
This is not really required: We check for some additional room, but we
never use it. The additional length is set to 0xa so the total length is
0xa + 8 = 18 which is fine with my 18 bytes.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>