bgpd: carry two MPLS labels in EVPN NLRIs

bgpd: carry two MPLS labels in EVPN NLRIs

When doing symmetric routing,
EVPN type-2 (MACIP) routes need to be advertised with two labels (VNIs)
the first being the L2 VNI (identifying the VLAN) and
the second being the L3 VNI (identifying the VRF).
The receive processing needs to handle one or two labels too.

Ticket: CM-18489
Review: CCR-6949
Testing: manual and bgp/evpn/mpls smoke

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>