net: stmmac: Set page_pool_params.max_len to a precise size

net: stmmac: Set page_pool_params.max_len to a precise size

DMA engine will always write no more than dma_buf_sz bytes of a received
frame into a page buffer, the remaining spaces are unused or used by CPU
exclusively.
Setting page_pool_params.max_len to almost the full size of page(s) helps
nothing more, but wastes more CPU cycles on cache maintenance.

For a standard MTU of 1500, then dma_buf_sz is assigned to 1536, and this
patch brings ~16.9% driver performance improvement in a TCP RX
throughput test with iPerf tool on a single isolated Cortex-A65 CPU
core, from 2.43 Gbits/sec increased to 2.84 Gbits/sec.

Signed-off-by: Furong Xu <0x1207@gmail.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Yanteng Si <si.yanteng@linux.dev>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>