uknetdev: Per device `lwip_data` to callbacks

uknetdev: Per device `lwip_data` to callbacks

In order to hand-over per device `lwip_data` to the rx/tx packet allocation
function (`netif_alloc_rxpkts()`), we move storing the reference to the
allocator that used for packet allocations to `lwip_data`. In the future,
this can be used to enable per device buffer pools (e.g.,
with `ukallocpool`) for faster allocation and freeing requests.

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Sharan Santhanam <sharan.santhanam@neclab.eu>