kfree_skb() handles a NULL skb argument so the additional check is
unnecessary. Remove it.

Link: https://lore.kernel.org/r/20200827092606.32148-1-vulab@iscas.ac.cnSigned-off-by: Xu Wang <vulab@iscas.ac.cn>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>