lib: Rework vrf_get

lib: Rework vrf_get

vrf_get has these possible lookup/creation ways of being called.

name = NULL, vrf_id = VRF_UNKNOWN
   Nothing to do here.

name = NULL, vrf_id != VRF_UNKNOWN
   Look up the vrf_id in the table.  Return that created vrf.
   If not found create a struct vrf to hold it properly.

name = <something>, vrf_id = VRF_UNKNOWN
   Look up the name in the vrf list, if found return that vrf.
   If not found create the vrf and zvrf

name = <something>, vrf_id != VRF_UNKNOWN
   Look up the name in the vrf list, if found check to see
   if the vrf->vrf_id is VRF_UNKNOWN, if so match up and return
   If not found, create the vrf and table entry and return

Ticket: CM-10139
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>
Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>