RDMA/nldev: Add NULL check to silence false warnings

Using nlmsg_put causes static analysis tools to many
false positives of not checking the return value of nlmsg_put.

In all uses in nldev.c, payload parameter is 0 so NULL will never
be returned. So let's add useless checks to silence the warnings.

Signed-off-by: Or Har-Toov <ohartoov@nvidia.com>
Reviewed-by: Michael Guralnik <michaelgur@nvidia.com>
Link: https://lore.kernel.org/r/bd924da89d5b4f5291a4a01d9b5ae47c0a9b6a3f.1669636336.git.leonro@nvidia.com


Signed-off-by: Leon Romanovsky <leon@kernel.org>