scsi: only maintain target_blocked if the driver has a target queue limit
scsi: only maintain target_blocked if the driver has a target queue limit

This saves us an atomic operation for each I/O submission and completion
for the usual case where the driver doesn't set a per-target can_queue
value.  Only a few iscsi hardware offload drivers set the per-target
can_queue value at the moment.

Signed-off-by: Christoph Hellwig <923f7720577207a44b32e59bbfbea59d27f1ae8e@lst.de>
Reviewed-by: Martin K. Petersen <0384aaef27f06874adbdb09a807bb339f4aff9fd@oracle.com>
Reviewed-by: Webb Scales <37ccb0fc6f308d8cc09a009b4a31ce019f4bec5c@hp.com>
Acked-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>
Tested-by: Bart Van Assche <89ed62d80e76c0eb24ee0d6433b48a91c2273b5e@acm.org>
Tested-by: Robert Elliott <fcf6aac10c16bf4310224494560ff8cb9772fa2e@hp.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs