Strict NLA validation is introduced from kernel version 4.20 where
NLA_UNSPEC usage is restricted. A new attibute type
NLA_POLICY_ETH_ADDR is introduced for MAC address.
Define a macro to use this if the kernel version is greater
than 4.19.
Also define NLA_EXACT_LEN as NLA_UNSPEC for kernels less
than 4.20.
Change-Id: I9d9bee7a12d84ca81b5777ccb417a33f08949203
CRs-Fixed: 2701357