RDMA/iwpm: Rely on the rdma_nl_[un]register() to ensure that requests are valid

RDMA/iwpm: Rely on the rdma_nl_[un]register() to ensure that requests are valid

The core netlink code alread guarentees that no netlink callback can be
running outside the rdma_nl_register/unregister() region and this
registration happens during module init/exit. Thus it is already prevented
that iwpm_valid_client() can ever fail. Remove it.

Link: https://lore.kernel.org/r/a9f05a78f9996bf6ea47099b5e02671bf742f5ab.1627048781.git.leonro@nvidia.com
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>