This change adds support for TSO of MPLS packets.

In my tests with tcpdump it seems to work. Note this test setup has
a 9000 byte MTU:

MPLS (label 100, exp 0, [S], ttl 64) IP srcip.50086 > dstip.1234:
  Flags [P.], seq 593345:644401, ack 0, win 420,
  options [nop,nop,TS val 45022534 ecr 1722291395], length 51056

IP dstip.1234 > srcip.50086: Flags [.], ack 593345, win 122,
  options [nop,nop,TS val 1722291395 ecr 45022534], length 0

IP dstip.1234 > srcip.50086: Flags [.], ack 602289, win 105,
  options [nop,nop,TS val 1722291395 ecr 45022534], length 0

IP dstip.1234 > srcip.50086: Flags [.], ack 620177, win 71,
  options [nop,nop,TS val 1722291395 ecr 45022534], length 0

MPLS (label 100, exp 0, [S], ttl 64) IP srcip.50086 > dstip.1234:
  Flags [P.], seq 644401:655953, ack 0, win 420,
  options [nop,nop,TS val 45022534 ecr 1722291395], length 11552

IP dstip.1234 > srcip.50086: Flags [.], ack 638065, win 37,
  options [nop,nop,TS val 1722291395 ecr 45022534], length 0

IP dstip.1234 > srcip.50086: Flags [.], ack 644401, win 25,
  options [nop,nop,TS val 1722291395 ecr 45022534], length 0

IP dstip.1234 > srcip.50086: Flags [.], ack 653345, win 8,
  options [nop,nop,TS val 1722291395 ecr 45022534], length 0

IP dstip.1234 > srcip.50086: Flags [.], ack 655953, win 3,
  options [nop,nop,TS val 1722291395 ecr 45022534], length 0
Signed-off-by: Joe Damato <jdamato@fastly.com>
Co-developed-by: Mike Gallo <mgallo@fastly.com>
Signed-off-by: Mike Gallo <mgallo@fastly.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>