app/testpmd: fix use of offload flags

In testpmd the rte_port->tx_ol_flags flag was used in 2 incompatible
manners:
- sometimes used with testpmd specific flags (0xff for checksums, and
  bit 11 for vlan)
- sometimes assigned to m->ol_flags directly, which is wrong in case
  of checksum flags

This commit replaces the hardcoded values by named definitions, which
are not compatible with mbuf flags. The testpmd forward engines are
fixed to use the flags properly.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>