target: Refactor MODE SENSE emulation

Convert spc_emulate_modesense() to use a table of mode pages, rather
than a switch statement.  This makes it possible to add more pages
sanely -- in particular we no longer need to make sure we keep the
0x3f (return all mode pages) case in sync.

While we're touching this code, make our MODE SENSE emulation a bit
better in a couple of ways:
 - When the initiator passes PC == 1 asking for changeable values,
   return all 0s to show we don't support setting anything.
 - Return a block descriptor for disk devices.

(nab: fix up device attribute references to use dev->dev_attrib
      in for-next code)

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