RDMA/nldev: Fix NULL pointer dereferences issue in rdma_nl_notify_event

nlmsg_put() may return a NULL pointer assigned to nlh, which will later
be dereferenced in nlmsg_end().

Fixes: 9cbed5aa ("RDMA/nldev: Add support for RDMA monitoring")
Link: https://patch.msgid.link/r/Zva71Yf3F94uxi5A@iZbp1asjb3cy8ks0srf007ZSigned-off-by: Qianqiang Liu <qianqiang.liu@163.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>

drivers/infiniband/core/nldev.c

@@ -2816,6 +2816,8 @@ int rdma_nl_notify_event(struct ib_device *device, u32 port_num,
 	nlh = nlmsg_put(skb, 0, 0,
 			RDMA_NL_GET_TYPE(RDMA_NL_NLDEV, RDMA_NLDEV_CMD_MONITOR),
 			0, 0);
+	if (!nlh)
+		goto err_free;
 
 	switch (type) {
 	case RDMA_REGISTER_EVENT: