packets: Always set ethertype in push_mpls()

packets: Always set ethertype in push_mpls()

There are two different MPLS ethertypes, 0x8847 and 0x8848 and a push MPLS
action applied to an MPLS packet may cause the ethertype to change from one
to the other. To ensure that this happens update the ethertype in
push_mpls() regardless of if the packet is already MPLS or not.

Test based on a similar test by Joe Stringer.

Cc: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>