scsi: scsi_debug: num_tgts must be >= 0

scsi: scsi_debug: num_tgts must be >= 0

BugLink: https://bugs.launchpad.net/bugs/1860816
[ Upstream commit aa5334c4f3014940f11bf876e919c956abef4089 ]

Passing the parameter "num_tgts=-1" will start an infinite loop that
exhausts the system memory

Link: https://lore.kernel.org/r/20191115163727.24626-1-mlombard@redhat.com
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>