scsi: Fix dm-multipath starvation when scsi host is busy
When a scsi host is kept busy by a dm-mpath device, other dm-mpath device
on the same host could be starved. For example:
It happens because dm-mpath delays request submission when the underlying
scsi host is busy even if sdev is not busy.
For case like this, it is better to send the request down and let scsi do
appropriate starvation control over the shared resource.
Though it might seem odd to change scsi's definition of a LLD being "busy",
it is reasonable because scsi_lld_busy (and blk_lld_busy) was introduced
to provide a hint for request-based stacking driver (i.e. dm-multipath)
and dm-multipath is the only user of this function.