datapath: Use strip_vlan() for modify_vlan_tci().

datapath: Use strip_vlan() for modify_vlan_tci().

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>