IB/srp: Fix a debug kernel crash

Avoid that the following BUG() is triggered against a debug
kernel:

kernel BUG at include/linux/scatterlist.h:92!
RIP: 0010:[<ffffffffa0467199>]  [<ffffffffa0467199>] srp_map_idb+0x199/0x1a0 [ib_srp]
Call Trace:
 [<ffffffffa04685fa>] srp_map_data+0x84a/0x890 [ib_srp]
 [<ffffffffa0469674>] srp_queuecommand+0x1e4/0x610 [ib_srp]
 [<ffffffff813f5a5e>] scsi_dispatch_cmd+0x9e/0x180
 [<ffffffff813f8b07>] scsi_request_fn+0x477/0x610
 [<ffffffff81298ffe>] __blk_run_queue+0x2e/0x40
 [<ffffffff81299070>] blk_delay_work+0x20/0x30
 [<ffffffff81071f07>] process_one_work+0x197/0x480
 [<ffffffff81072239>] worker_thread+0x49/0x490
 [<ffffffff810787ea>] kthread+0xea/0x100
 [<ffffffff8159b632>] ret_from_fork+0x22/0x40

Fixes: f7f7aab1a5c0 ("IB/srp: Convert to new registration API")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Christoph Hellwig <hch@lst.de>
Cc: <stable@vger.kernel.org> # v4.4+
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/ulp/srp/ib_srp.c

@@ -1501,7 +1501,7 @@ static int srp_map_idb(struct srp_rdma_ch *ch, struct srp_request *req,
 
 	if (dev->use_fast_reg) {
 		state.sg = idb_sg;
-		sg_set_buf(idb_sg, req->indirect_desc, idb_len);
+		sg_init_one(idb_sg, req->indirect_desc, idb_len);
 		idb_sg->dma_address = req->indirect_dma_addr; /* hack! */
 #ifdef CONFIG_NEED_SG_DMA_LENGTH
 		idb_sg->dma_length = idb_sg->length;	      /* hack^2 */