ip_frag: remove IP checkum offload flag

ip_frag: remove IP checkum offload flag

Currently PKT_TX_IP_CKSUM is being set into mbuf->ol_flags
during fragmentation and reassemble operation implicitly.
Because of this, application is forced to use checksum offload
whether it is supported by platform or not.

Also documentation does not provide any expected value of ol_flags
in returned mbuf (reassembled or fragmented) so application will never
come to know that which offloads are enabled. So transmission may be failed
for the platforms which does not support checksum offload.

Also, IPv6 does not contain any checksum field in header so setting
mbuf->ol_flags with PKT_TX_IP_CKSUM is itself invalid.

So removing mentioned flag from the library.

Signed-off-by: Sunil Kumar Kori <skori@marvell.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>