igb: Add support for persistent NAPI config

Use netif_napi_add_config() to assign persistent per-NAPI config.

This is useful for preserving NAPI settings when changing queue counts or
for user space programs using SO_INCOMING_NAPI_ID.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Rinitha S <sx.rinitha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>