net: tap: check vlan with eth_type_vlan() method

Replace some checks for ETH_P_8021Q and ETH_P_8021AD in
drivers/net/tap.c with eth_type_vlan.

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Link: https://lore.kernel.org/r/20210115023238.4681-1-dong.menglong@zte.com.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>