As the nla_nest_start() may fail with NULL returned, the return value needs
to be checked.

Fixes: c4ffee7c ("RDMA/netlink: Implement counter dumpit calback")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Link: https://lore.kernel.org/r/20221126043410.85632-1-yuancan@huawei.comSigned-off-by: Leon Romanovsky <leon@kernel.org>