net: stmmac: rearrange RX buffer allocation and free functions

This patch restructures the per RX queue buffer allocation from page_pool
to stmmac_alloc_rx_buffers().

We also rearrange dma_free_rx_skbufs() so that it can be used in
init_dma_rx_desc_rings() during freeing of RX buffer in the event of
page_pool allocation failure to replace the more efficient method earlier.
The replacement is needed to make the RX buffer alloc and free method
scalable to XDP ZC xsk_pool alloc and free later.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>