net: ethernet: mtk_eth_soc: fix possible NULL pointer dereference in mtk_hwlro_get_fdir_all()

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

net: ethernet: mtk_eth_soc: fix possible NULL pointer dereference in mtk_hwlro_get_fdir_all()

rule_locs is allocated in ethtool_get_rxnfc and the size is determined by

rule_cnt from user space. So rule_cnt needs to be check before using

rule_locs to avoid NULL pointer dereference.