mlx4_core: Expose a few helpers to fill DMFS HW strucutures

Re-arrange some of code which fills DMFS HW structures so we can use
it from within the core driver and from the IB driver too, e.g when
verbs DMFS structures are transformed into mlx4 hardware structs.

Also, add struct mlx4_flow_handle struct which will be of use by the
DMFS verbs flow in the IB driver.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>