IB/srp: Avoid duplicate devices from LUN scan

IB/srp: Avoid duplicate devices from LUN scan

SCSI scanning of a channel:id:lun triplet in Linux works as follows
(function scsi_scan_target() in drivers/scsi/scsi_scan.c):

- If lun == SCAN_WILD_CARD, send a REPORT LUNS command to the target
  and process the result.

- If lun != SCAN_WILD_CARD, send an INQUIRY command to the LUN
  corresponding to the specified channel:id:lun triplet to verify
  whether the LUN exists.

So a SCSI driver must either take the channel and target id values in
account in its quecommand() function or it should declare that it only
supports one channel and one target id.

Currently the ib_srp driver does neither.  As a result scanning the
SCSI bus via e.g. rescan-scsi-bus.sh causes many duplicate SCSI
devices to be created. For each 0:0:L device, several duplicates are
created with the same LUN number and with (C:I) != (0:0). Fix this by
declaring that the ib_srp driver only supports one channel and one
target id.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: <stable@kernel.org>
Acked-by: David Dillow <dillowda@ornl.gov>
Signed-off-by: Roland Dreier <roland@purestorage.com>