[SCSI] scsi_lib: use blk_rq_tagged in scsi_request_fn

I goofed and did not see the macro for checking if a request is tagged.
This patch has us use blk_rq_tagged instead of digging into the req->tag.

Patch was made over scsi-misc.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>