net: axienet: Use napi_alloc_skb when refilling RX ring

Use napi_alloc_skb to allocate memory when refilling the RX ring
in axienet_poll for more efficiency. napi_alloc_skb() can reuse
softirq-local cache of freed skbs which may still be cache-warm
and skipping allocator calls.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Link: https://lore.kernel.org/r/20220308211013.1530955-1-robert.hancock@calian.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>