bgmac: reduce max frame size to support just MTU 1500

bgmac: reduce max frame size to support just MTU 1500

bgmac allocates new replacement buffer before handling each received
frame. Allocating & DMA-preparing 9724 B each time consumes a lot of CPU
time. Ideally bgmac should just respect currently set MTU but it isn't
the case right now. For now just revert back to the old limited frame
size.

This change bumps NAT masquerade speed by ~95%.

Since commit 8218f62c9c9b ("mm: page_frag: use initial zero offset for
page_frag_alloc_align()"), the bgmac driver fails to open its network
interface successfully and runs out of memory in the following call
stack:

bgmac_open
  -> bgmac_dma_init
    -> bgmac_dma_rx_skb_for_slot
      -> netdev_alloc_frag

BGMAC_RX_ALLOC_SIZE = 10048 and PAGE_FRAG_CACHE_MAX_SIZE = 32768.

Eventually we land into __page_frag_alloc_align() with the following
parameters across multiple successive calls:

__page_frag_alloc_align: fragsz=10048, align_mask=-1, size=32768, offset=0
__page_frag_alloc_align: fragsz=10048, align_mask=-1, size=32768, offset=10048
__page_frag_alloc_align: fragsz=10048, align_mask=-1, size=32768, offset=20096
__page_frag_alloc_align: fragsz=10048, align_mask=-1, size=32768, offset=30144

So in that case we do indeed have offset + fragsz (40192) > size (32768)
and so we would eventually return NULL. Reverting to the older 1500
bytes MTU allows the network driver to be usable again.

Fixes: 8c7da63978f1 ("bgmac: configure MTU and add support for frames beyond 8192 byte size")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
[florian: expand commit message about recent commits]
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20250127175159.1788246-1-florian.fainelli@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>