Zebra: Handle VxLAN encap in netlink rtmsg for FPM

Zebra: Handle VxLAN encap in netlink rtmsg for FPM

- For data plane processing of VxLAN routes, add encap type and L3VNI info to
  rtmsg message for FPM.
- Add "RTA_ENCAP_TYPE" attribute for VxLAN encap with value 100.
  This value is not currently used for RTA_ENCAP_TYPE for any encap.
- If "RTA_ENCAP_TYPE" is 100, add "RTA_ENCAP" attribute with "RTA_VNI" as a
  nested attribute of RTA_ENCAP

Format of RTA_VNI attribute:
Len(2 bytes)       type (2 bytes)      Value(4 bytes)(VNI)
   00    08     :     00    00     :      1000

RTA_VNI attribute is a custom attribute.

Signed-off-by: Ameya Dharkar <adharkar@vmware.com>