zebra: resolve issue with importing table entries correctly

zebra: resolve issue with importing table entries correctly

Problem reported when a table entry originated by rdnbrd was moved from one
interface to another on the same switch.  Both would be deleted, leaving
no imported entry in the table.  Modified zebra_add_import_table_entry to
used rib_add_multipath as well as correct the call to delete a duplicate
entry to include the nexthop associated with the route_entry.

Ticket: CM-18154
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed By: CCR-6731
Testing Done: Manual testing successful, deb given to submitter, bgp-smoke
had no new failures