net/bonding: Take IP hash logic into a helper

Hash logic on L3 will be used in a downstream patch for one more use
case.
Take it to a function for a better code reuse.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Boris Pismenny <borisp@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>