net: libwx: fix the using of Rx buffer DMA

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

net: libwx: fix the using of Rx buffer DMA

The wx_rx_buffer structure contained two DMA address fields: 'dma' and 'page_dma'. However, only 'page_dma' was actually initialized and used to program the Rx descriptor. But 'dma' was uninitialized and used in some paths.

This could lead to undefined behavior, including DMA errors or use-after-free, if the uninitialized 'dma' was used. Althrough such error has not yet occurred, it is worth fixing in the code.