RDMA/siw: Fix potential siw_mem refcnt leak in siw_fastreg_mr()

siw_fastreg_mr() invokes siw_mem_id2obj(), which returns a local reference
of the siw_mem object to "mem" with increased refcnt.  When
siw_fastreg_mr() returns, "mem" becomes invalid, so the refcount should be
decreased to keep refcount balanced.

The issue happens in one error path of siw_fastreg_mr(). When "base_mr"
equals to NULL but "mem" is not NULL, the function forgets to decrease the
refcnt increased by siw_mem_id2obj() and causes a refcnt leak.

Reorganize the flow so that the goto unwind can be used as expected.

Fixes: b9be6f18 ("rdma/siw: transmit path")
Link: https://lore.kernel.org/r/1586939949-69856-1-git-send-email-xiyuyang19@fudan.edu.cnReported-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/sw/siw/siw_qp_tx.c

@@ -920,20 +920,27 @@ static int siw_fastreg_mr(struct ib_pd *pd, struct siw_sqe *sqe)
 {
 	struct ib_mr *base_mr = (struct ib_mr *)(uintptr_t)sqe->base_mr;
 	struct siw_device *sdev = to_siw_dev(pd->device);
-	struct siw_mem *mem = siw_mem_id2obj(sdev, sqe->rkey  >> 8);
+	struct siw_mem *mem;
 	int rv = 0;
 
 	siw_dbg_pd(pd, "STag 0x%08x\n", sqe->rkey);
 
-	if (unlikely(!mem || !base_mr)) {
+	if (unlikely(!base_mr)) {
 		pr_warn("siw: fastreg: STag 0x%08x unknown\n", sqe->rkey);
 		return -EINVAL;
 	}
+
 	if (unlikely(base_mr->rkey >> 8 != sqe->rkey  >> 8)) {
 		pr_warn("siw: fastreg: STag 0x%08x: bad MR\n", sqe->rkey);
-		rv = -EINVAL;
-		goto out;
+		return -EINVAL;
 	}
+
+	mem = siw_mem_id2obj(sdev, sqe->rkey  >> 8);
+	if (unlikely(!mem)) {
+		pr_warn("siw: fastreg: STag 0x%08x unknown\n", sqe->rkey);
+		return -EINVAL;
+	}
+
 	if (unlikely(mem->pd != pd)) {
 		pr_warn("siw: fastreg: PD mismatch\n");
 		rv = -EINVAL;