datapath: Add compatibility with sk_buff's vlan_tci before 2.6.33.

datapath: Add compatibility with sk_buff's vlan_tci before 2.6.33.

Between 2.6.27 and 2.6.32, the vlan_tci member of struct sk_buff was the
raw value of the 802.1Q header's TCI field, without the CFI bit being set.
In 2.6.33 and later, the CFI bit is always set if an 802.1Q header is
present, correcting a corner case.

Until now, OVS has not consistently dealt with this.  If a packet arrived
at a datapath from a network device directly, or if it was set with an
ODP_ACTION_ATTR_SET_DL_TCI action, then the CFI bit would not be set in
vlan_tci.  In flow_extract(), OVS copies vlan_tci directly to dl_tci in the
flow structure (via vlan_get_tci()), so the CFI bit would also not be set
in dl_tci.  But if OVS had to send a packet up to userspace (converting the
vlan_tci back to an 802.1Q header along the way) and got it back, then it
would set the VLAN CFI bit in dl_tci when it parsed the 802.1Q header in
parse_vlan().  This had the effect that a flow set up by userspace (with
the CFI bit set) would never be matched by a packet arriving from a network
device, because they would have different dl_tci values.

This fixes the problem, by making the vlan_get_tci() and vlan_set_tci()
interface consistent across kernel versions.  Now, they always accept or
return a value where the VLAN CFI bit is set if an 802.1Q header is
present.

Build-tested only.

Problem isolated by Ethan Jackson <ethan@nicira.com>

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Reported-by: Ram Jothikumar <rjothikumar@nicira.com>
Bug #4915.