erspan: set bso when truncated bit is set.

erspan: set bso when truncated bit is set.

Before the patch, the erspan BSO bit (Bad/Short/Oversized) is not
handled.  BSO has 4 possible values:
  00 --> Good frame with no error, or unknown integrity
  11 --> Payload is a Bad Frame with CRC or Alignment Error
  01 --> Payload is a Short Frame
  10 --> Payload is an Oversized Frame

This patch set BSO to 01 when truncate is true.

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>