net/skbuff: add macros for VLAN_PRESENT bit

Wrap VLAN_PRESENT bit using macro like PKT_TYPE_* and CLONED_*,
as used by BPF code.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>