scsi: scsi_debug: Add two new parameters to scsi_debug driver

This patch adds two new parameters to the scsi_debug driver.

During various fault injection scenarios it would be useful to be able
to pick a specific starting sector and number of follow on sectors where
a MEDIUM ERROR for reads would be returned against a scsi-debug device.

Right now this only works against sector 0x1234 and OPT_MEDIUM_ERR_NUM
follow on sectors.  However during testing of md-raid and other
scenarios I wanted more flexibility.

The idea is add 2 new parameters:

    medium_error_start
    medium_error_count

If medium_error_start is set then we don't use the default of
OPT_MEDIUM_ERR_ADDR, but use that set value.

If medium_error_count is set we use that value otherwise default to
OPT_MEDIUM_ERR_NUM.

Signed-off-by: Laurence Oberman <loberman@redhat.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>