rules 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
rules to avoid OOB writing or NULL pointer dereference.

Fixes: 90b509b3 ("net: mvpp2: cls: Add Classification offload support")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Reviewed-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>