tunnels: Don't initialize unnecessary packet metadata.

tunnels: Don't initialize unnecessary packet metadata.

The addition of Geneve options to packet metadata significantly
expanded its size. It was reported that this can decrease performance
for DPDK ports by up to 25% since we need to initialize the whole
structure on each packet receive.

It is not really necessary to zero out the entire structure because
miniflow_extract() only copies the tunnel metadata when particular
fields indicate that it is valid. Therefore, as long as we zero out
these fields when the metadata is initialized and ensure that the
rest of the structure is correctly set in the presence of a tunnel,
we can avoid touching the tunnel fields on packet reception.

Reported-by: Ciara Loftus <ciara.loftus@intel.com>
Tested-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>