[SCSI] initialize max_target_blocked in scsi_alloc_target

This patch initializes the max_target_blocked field of a scsi target
structure so that a queuecommand return value of
SCSI_MLQUEUE_TARGET_BUSY will actually result in having the
scsi_queue_insert blocking the device queue before requeuing the
command and running the queue.  Otherwise, can and does cause livelock
on single CPU configurations if/when open-iSCSI software initiator's
command PDU window fills.

Signed-off-by: Ed Goggin <egoggin@vmware.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>