mbuf: clarify QinQ flag usage

Update implementation that when PKT_RX_QINQ_STRIPPED mbuf ol_flags
set by PMD, PKT_RX_QINQ, PKT_RX_VLAN_STRIPPED & PKT_RX_VLAN
should be also set.

Clarify mbuf documentations that when PKT_RX_QINQ set PKT_RX_VLAN also
should be set.

So that appllication can rely on PKT_RX_QINQ flag to access both
mbuf.vlan_tci & mbuf.vlan_tci_outer

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>