mbuf: remove Intel offload checks from generic API

mbuf: remove Intel offload checks from generic API

rte_validate_tx_offload() is used in Tx prepare callbacks
(RTE_LIBRTE_ETHDEV_DEBUG only) to check Tx offloads consistency.
Requirement that packet headers should not be fragmented is not
documented and unclear where it comes from except
rte_net_intel_cksum_prepare() functions which relies on it.

It could be NIC vendor specific driver or hardware limitation, but,
if so, it should be documented and checked in corresponding Tx
prepare callbacks.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>