Is it considered stad-padding if player A sees that player B has nice dogtag he wants to get. Also if the player B agrees to give the dogtag to player A by letting him kill for free if the player A gives player B something back in return.

Stat-padding is just that - building up stats in an unfair way that aren't meant to be done by the game design so to speak. Like killing an enemy multiple times to unlock things while the enemy doesn't shoot back at you and doesn't move, thus fluffing the numbers like kpm, accuracy, hs rate and such (because by the game design an enemy is SUPPOSED to shoot back and try to take you down and your accuracy on guns isn't meant to be higher a certain value, same for hs rate). For little things like unlocking special weapons (like Unica/deagle for example, or what you just described like getting a few dog tags) it is allowed to do so but not more than that. After you've got the weapon you must play it fairly. 

http://tos.ea.com/legalapp/ROC/US/en/PC/BF4

I don't know if other admins or other people have another better explanation of course but bf4db would certainty not ban someone over the agreement to get a few dogtags

Just don't go for headshots....

Ofcourse not. You cant get a dogtag if you headshot it would only work with melee from behind.