IB/mlx5: Add drop flow steering rule support

A drop rule is described by an action drop and no destination.
If a user specified IB_FLOW_SPEC_ACTION_DROP then set the action
to MLX5_FLOW_CONTEXT_ACTION_DROP and clear the destination.

Signed-off-by: Slava Shwartsman <slavash@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>