RDMA/nldev: Prepare CAP_NET_ADMIN checks for .doit callbacks

The .doit callbacks don't have a netlink_callback to check capabilities so
in order to use the same fill_res_func for both .dump and .doit, we need
to do the capability check outside of those functions.

For .doit callbacks, it is possible to check CAP_NET_ADMIN directly on the
received sk_buff.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>