The sematics for setting a vlan tag are to modify the existing tag
if one exists. This can be expressed as removing the existing tag
first and then adding a new one. This simplifies the code by not
requiring two copies of the logic that manipulates non-accelerated
vlans and should not make a performance difference because the vlan
tag is contained in a single cache line.
Signed-off-by: Jesse Gross <jesse@nicira.com> Acked-by: Ben Pfaff <blp@nicira.com>