net: fec: check the return value of build_skb()

net: fec: check the return value of build_skb()

The build_skb might return a null pointer but there is no check on the
return value in the fec_enet_rx_queue(). So a null pointer dereference
might occur. To avoid this, we check the return value of build_skb. If
the return value is a null pointer, the driver will recycle the page and
update the statistic of ndev. Then jump to rx_processing_done to clear
the status flags of the BD so that the hardware can recycle the BD.

Fixes: 95698ff6177b ("net: fec: using page pool to manage RX buffers")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Shenwei Wang <Shenwei.wang@nxp.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://lore.kernel.org/r/20221219022755.1047573-1-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>