net: skbuff: rename __pkt_vlan_present_offset to __mono_tc_offset

vlan_present is gone since
commit 354259fa ("net: remove skb->vlan_present")
rename the offset field to what BPF is currently looking
for in this byte - mono_delivery_time and tc_at_ingress.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230321014115.997841-2-kuba@kernel.org


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>