target: Set peripheral device type consistently in INQUIRY response

Current code sets the peripheral device type to 0x3f == "not present
unknown" for virtual LUN 0 for standard INQUIRY commands, but leaves it
as 0 == "connected direct access block" for VPD INQUIRY commands.  This
is just because the check for LUN 0 only happens in some code paths.

Make our peripheral device type consistent by moving the LUN 0 check
into the common emulate_inquiry() code.

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