tun: reserve extra headroom only when XDP is set

We reserve headroom unconditionally which could cause unnecessary
stress on socket memory accounting because of increased trusesize. Fix
this by only reserve extra headroom when XDP is set.

Cc: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>