bonding: Add helper function to get the xmit slave based on hash

Both xor and 802.3ad modes use bond_xmit_hash to get the xmit slave.
Export the logic to helper function so it could be used in the
following patches by the .ndo to get the xmit slave.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>