IB/core: Fix deadlock on uverbs modify_qp error flow

The deadlock occurs in __uverbs_modify_qp: we take a lock (idr_read_qp)
and in case of failure in ib_resolve_eth_l2_attrs we don't release
it (put_qp_read).  Fix that.

Fixes: ed4c54e5 ("IB/core: Resolve Ethernet L2 addresses when modifying QP")
Signed-off-by: Moshe Lazer <moshel@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/core/uverbs_cmd.c

@@ -2091,20 +2091,21 @@ ssize_t ib_uverbs_modify_qp(struct ib_uverbs_file *file,
 	if (qp->real_qp == qp) {
 		ret = ib_resolve_eth_l2_attrs(qp, attr, &cmd.attr_mask);
 		if (ret)
-			goto out;
+			goto release_qp;
 		ret = qp->device->modify_qp(qp, attr,
 			modify_qp_mask(qp->qp_type, cmd.attr_mask), &udata);
 	} else {
 		ret = ib_modify_qp(qp, attr, modify_qp_mask(qp->qp_type, cmd.attr_mask));
 	}
 
-	put_qp_read(qp);
-
 	if (ret)
-		goto out;
+		goto release_qp;
 
 	ret = in_len;
 
+release_qp:
+	put_qp_read(qp);
+
 out:
 	kfree(attr);