mbuf: extend meaning of QinQ stripped bit

Certain hardware may be able to strip and/or save only the outermost
VLAN instead of both the VLANs in the mbuf in a QinQ scenario.
To handle such cases, we could re-interpret setting of just
PKT_RX_QINQ_STRIPPED to indicate that only the outermost VLAN has
been stripped and saved in mbuf->vlan_tci_outer.
Only When both PKT_RX_QINQ_STRIPPED and PKT_RX_VLAN_STRIPPED are set, the 2
VLANs have been stripped by the hardware and their TCI are saved in
mbuf->vlan_tci (inner) and mbuf->vlan_tci_outer (outer).

Signed-off-by: Somnath Kotur <somnath.kotur@broadcom.com>