net: stmmac: set PP_FLAG_DMA_SYNC_DEV only if XDP is enabled

net: stmmac: set PP_FLAG_DMA_SYNC_DEV only if XDP is enabled

Commit 5fabb01207a2 ("net: stmmac: Add initial XDP support") sets
PP_FLAG_DMA_SYNC_DEV flag for page_pool unconditionally,
page_pool_recycle_direct() will call page_pool_dma_sync_for_device()
on every page even the page is not going to be reused by XDP program.

When XDP is not enabled, the page which holds the received buffer
will be recycled once the buffer is copied into new SKB by
skb_copy_to_linear_data(), then the MAC core will never reuse this
page any longer. Always setting PP_FLAG_DMA_SYNC_DEV wastes CPU cycles
on unnecessary calling of page_pool_dma_sync_for_device().

After this patch, up to 9% noticeable performance improvement was observed
on certain platforms.

Fixes: 5fabb01207a2 ("net: stmmac: Add initial XDP support")
Signed-off-by: Furong Xu <0x1207@gmail.com>
Link: https://patch.msgid.link/20240919121028.1348023-1-0x1207@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>