mbuf: enforce semantics for Tx inline IPsec processing

mbuf: enforce semantics for Tx inline IPsec processing

Not all net PMD's/HW can parse packet and identify L2 header and
L3 header locations on Tx. This is inline with other Tx offloads
requirements such as L3 checksum, L4 checksum offload, etc,
where mbuf.l2_len, mbuf.l3_len etc, needs to be set for HW to be
able to generate checksum. Since Inline IPsec is also such a Tx
offload, some PMD's at least need mbuf.l2_len to be valid to
find L3 header and perform Outbound IPSec processing.

Hence, this patch updates documentation to enforce setting
mbuf.l2_len while setting PKT_TX_SEC_OFFLOAD in mbuf.ol_flags
for Inline IPsec Crypto / Protocol offload processing to
work on Tx.

Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <gakhil@marvell.com>