rdma: Unify netlink attribute checks prior to prints

rdma: Unify netlink attribute checks prior to prints

Place check if netlink attribute available in general place,
instead of doing the same check in many paces.

Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>