net: mvpp2: limit XDP frame size to the RX buffer

net: mvpp2: limit XDP frame size to the RX buffer

mvpp2 has short and long BM pools, and short pool buffers can be smaller
than PAGE_SIZE. The XDP path nevertheless initializes every xdp_buff with
PAGE_SIZE as frame size.

XDP helpers use frame_sz to validate tail growth and to derive the hard
end of the data area. Advertising PAGE_SIZE for short buffers can let
bpf_xdp_adjust_tail() grow a packet past the real allocation, corrupting
memory or later tripping skb tailroom checks.

Initialize the XDP buffer with bm_pool->frag_size so XDP tailroom matches
the actual buffer backing the packet.

Fixes: 07dd0a7aae7f ("mvpp2: add basic XDP support")
Signed-off-by: Til Kaiser <mail@tk154.de>
Link: https://patch.msgid.link/20260607134943.21996-3-mail@tk154.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>