{pktgen, xfrm} Update IPv4 header total len and checksum after tranformation

{pktgen, xfrm} Update IPv4 header total len and checksum after tranformation

commit a553e4a6317b2cfc7659542c10fe43184ffe53da ("[PKTGEN]: IPSEC support")
tried to support IPsec ESP transport transformation for pktgen, but acctually
this doesn't work at all for two reasons(The orignal transformed packet has
bad IPv4 checksum value, as well as wrong auth value, reported by wireshark)

- After transpormation, IPv4 header total length needs update,
  because encrypted payload's length is NOT same as that of plain text.

- After transformation, IPv4 checksum needs re-caculate because of payload
  has been changed.

With this patch, armmed pktgen with below cofiguration, Wireshark is able to
decrypted ESP packet generated by pktgen without any IPv4 checksum error or
auth value error.

pgset "flag IPSEC"
pgset "flows 1"

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>