IB/core: Introduce drop flow specification

This flow steering specification identifies flow for drop by the HW.
If user create a flow only with the drop specification,
then all the packets that hit this flow will be dropped, otherwise the HW
will drop only the packets that match the other L2/L3/L4 specifications.

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>