fix: now we are encode label according to rfc3107 (with BOS);

little fix of label encoding in l2vpn nlri. now we follows rfc3107 and encode label with BOS bit
"
Each label is encoded as 3
         octets, where the high-order 20 bits contain the label value,
         and the low order bit contains "Bottom of Stack" (as defined in
         [MPLS-ENCAPS]).
"