kni: use dedicated function to set random MAC address

kni: use dedicated function to set random MAC address

eth_hw_addr_random() sets address type correctly.

eth_hw_addr_random() is available since Linux v3.4, so
no compat is required.

Also fix the warning:
warning: passing argument 1 of ‘memcpy’ discards ‘const’
qualifier from pointer target type

Variable dev_addr is done const intentionally in Linux v5.17 to
prevent using it directly.

Fixes: ea6b39b5b847 ("kni: remove ethtool support")
Cc: stable@dpdk.org
Signed-off-by: Ke Zhang <ke1x.zhang@intel.com>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Ferruh Yigit <ferruh.yigit@xilinx.com>