[SCSI] iscsi: lower queue depth

From Wang Zhenyu:

High queue depth was a problem for some targets so make queue_depth adjustable

From Mike Christie

Make default queue_depth a little lower

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Alex Aizman <itn780@yahoo.com>
Signed-off-by: Dmitry Yusupov <dmitry_yus@yahoo.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>