zebra: resolve issue with rnh not evaluating nexhops correctly

zebra: resolve issue with rnh not evaluating nexhops correctly

Problem discovered in testing that occasionally when an interface
address was flushed, the corresponding route would be removed from
the kernel and zebra but remain in the bgp table and be advertised
to peers.  Discovered that when zebra_rib_evaluate_nexthops spun
thru the tree list of rns, if the timing and circumstances were
right, it would move elements and miss evaluating some.  Changed
from frr_each to frr_each_safe and the problem is now gone.

Ticket: CM-25301
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>