[SCSI] tgt: fix warning

Using scsi_tgt_lib in a new target module, we were getting
the following warning and a stack traceback on every I/O completion:

WARNING: at block/blk-core.c:1108

Which is claiming we may be leaking a bio.
We don't leak bios (blk_rq_unmap_user should free them).

Set rq->bio to NULL before calling scsi_host_put_command().
This was as advised by Fujita Tomonori.
Signed-off-by: NJoe Eykholt <jeykholt@cisco.com>
Acked-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: NJames Bottomley <James.Bottomley@suse.de>

drivers/scsi/scsi_tgt_lib.c

@@ -185,6 +185,7 @@ static void scsi_tgt_cmd_destroy(struct work_struct *work)
 	dprintk("cmd %p %d %u\n", cmd, cmd->sc_data_direction,
 		rq_data_dir(cmd->request));
 	scsi_unmap_user_pages(tcmd);
+	tcmd->rq->bio = NULL;
 	scsi_host_put_command(scsi_tgt_cmd_to_host(cmd), cmd);
 }