net/mlx4_core: Add helper function to translate B0 steering rules to DMFS

A pre-step for supporting guests that use B0 steering over a hypervisor
that runs in DMFS (device managed flow steering mode). Add helper function
which allows to translate L2 attachments / detachments provided in B0 mode
to DMFS rules.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>