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@iZbp1asjb3cy8ks0srf007Z


Signed-off-by: Qianqiang Liu <qianqiang.liu@163.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>