net: gro: no longer use skb_vlan_tag_present()

We can remove a conditional test in gro_list_prepare()
by comparing vlan_all fields of the two skbs.

Notes:

While comparing the vlan_proto is not strictly needed,
because part of the following compare_ether_header() call,
using 32bit word is actually faster than using 16bit values.

napi_reuse_skb() makes sure to clear skb->vlan_all,
as it already calls __vlan_hwaccel_clear_tag()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>