Combine first 8 ingredients in a large heavy-duty, zip-top plastic bag; add salmon steaks. Seal bag, and marinate steaks in refrigerator 2 hours, turning bag occasionally. Remove salmon steaks from marinade, reserving marinade.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place steaks on rack; grill, uncovered, 5 to 6 minutes on each side or until fish flakes easily when tested with a fork, basting frequently with reserved marinade.
Combine mayonnaise and next 3 ingredients, stirring well. Top each salmon steak with 1 tablespoon mayonnaise mixture. Garnish with fresh cilantro, if desired.