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

In the Linux kernel, the following vulnerability has been resolved: 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.