datapath: Don't drop MTU-sized VLAN packets from userspace

datapath: Don't drop MTU-sized VLAN packets from userspace

Before transimitting a packet, the datapath checks that the packet
length is not greater than the MTU.  It determines the length based on
the 'protocol' field in the skb.  If 'protocol' is ETH_P_8021Q, it reduces
the packet length as stored in the 'len' field by four bytes, which
is the size of a VLAN tag header.  Unfortunately, packets that arrived
from userspace were not having the 'protocol' field set, which would
cause MTU-sized packets to be dropped.  This commit sets the 'protocol'
field appropriately.

Thanks to Ben Pfaff for the help diagnosing this issue.

NIC-17 and NIC-26