IB/srp: Add target_can_queue login parameter

Although I'm not sure this parameter is useful for regular SRP users,
setting this parameter to 1 has shown to be invaluable for testing the
block layer core, SCSI core and device mapper queue running mechanisms.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>