RDMA/netlink: Simplify netlink listener existence check

All users of rdma_nl_chk_listeners() are interested to get boolean answer
if netlink socket has listeners, so update all places to boolean function.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>