target: Add emulation for MODE SELECT

This is another thing that compliance tests try, and it's easy to
implement on top of the MODE SENSE refactoring; since we don't claim
to support any changeable values, all we need to do is check that
the page contents sent by the initiator match what we would return.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>