zebra: nhg code selection doesnot check invalid status

Problem description:
1)In some corner cases in zebra, “Failed to enqueue dataplane  install”
happened for routes which was caused because of INVALID nhg.
2)Our NHG code logic, check all nexthop entries is valid or not,but it
does not check whether the nhg itself is valid or not during our
checks which in turn cause route install failures in zebra.

Fix:
Handled the nhg logic to check for validity during selection.

Signed-off-by: harini <[email protected]>

Author: hnattamaisub

zebra/zebra_rib.c

@@ -1379,6 +1379,10 @@ static void rib_process(struct route_node *rn)
 				continue;
 			}
 		} else {
+			/* Skip routes with invalid NHGs though re has not changed nhg
+			 * might be not valid during interface flap events */
+			if (re->nhe && !CHECK_FLAG(re->nhe->flags, NEXTHOP_GROUP_VALID))
+				continue;
 			/*
 			 * If the re has not changed and the nhg we have is
 			 * not usable, then we cannot use this route entry