RDMA/netlink: Add __maybe_unused to static inline in C file

RDMA/netlink: Add __maybe_unused to static inline in C file

Like other commits in the tree add __maybe_unused to a static inline in a
C file because some clang compilers will complain about unused code:

>> drivers/infiniband/core/nldev.c:2543:1: warning: unused function '__chk_RDMA_NL_NLDEV'
   MODULE_ALIAS_RDMA_NETLINK(RDMA_NL_NLDEV, 5);
   ^

Fixes: e3bf14bdc17a ("rdma: Autoload netlink client modules")
Link: https://lore.kernel.org/r/4a8101919b765e01d7fde6f27fd572c958deeb4a.1636267207.git.leonro@nvidia.com
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>