zebra: Fix another ships in the night issue with WFI

zebra: Fix another ships in the night issue with WFI

Effectively When bgp would send a route update down
to zebra and immediately after that a asic update
from the kernel was read.  Zebra would choose the
asic update and drop the bgp update leaving us in
a state where bgp was not used as the true source.

Modify the code so that in rib_multipath_nhe
we notice that we have an unprocessed route update
from bgp.  And if so just drop this kernel update
about an older version of the route since it is
no longer needed.

Ticket: 2722533
Signed-off-by: Donald Sharp <sharpd@nvidia.com>