ipam/allocator: Fix nil check on node CIDR

Previously, the nil check was useless because the variable is never
going to be nil, as it was initialized to an empty `nodeCIDRs` instance.
This made the "Unable to allocate node CIDR" error impossible to
surface.

This commit fixes this by making the variable not a pointer, but rather
a value, and reworking the further checks to ensure that the error in
allocating a node CIDR can be surfaced.

Fixes: ef6ecbd ("add error log When ipam allocate nodecidr failure")
Fixes: #13299

Signed-off-by: Chris Tarazi <chris@isovalent.com>

pkg/ipam/allocator/podcidr/podcidr.go

@@ -849,42 +849,46 @@ func (n *NodesPodCIDRManager) allocateNext(nodeName string) (*nodeCIDRs, bool, e
 		}
 	}()
 
-	var v4CIDR, v6CIDR *net.IPNet
-
-	nCIDRs := &nodeCIDRs{}
+	var (
+		cidrs          nodeCIDRs
+		v4CIDR, v6CIDR *net.IPNet
+	)
 
 	// Only allocate a v4 CIDR if the v4CIDR allocator is available
 	if len(n.v4CIDRAllocators) != 0 {
 		revertFunc, v4CIDR, err = allocateFirstFreeCIDR(n.v4CIDRAllocators)
 		if err != nil {
 			return nil, false, err
 		}
-		revertStack.Push(revertFunc)
+
 		log.WithField("CIDR", v4CIDR).Debug("v4 allocated CIDR")
-		nCIDRs.v4PodCIDRs = []*net.IPNet{v4CIDR}
+		cidrs.v4PodCIDRs = []*net.IPNet{v4CIDR}
+
+		revertStack.Push(revertFunc)
 	}
 	if len(n.v6CIDRAllocators) != 0 {
 		revertFunc, v6CIDR, err = allocateFirstFreeCIDR(n.v6CIDRAllocators)
 		if err != nil {
 			return nil, false, err
 		}
-		revertStack.Push(revertFunc)
+
 		log.WithField("CIDR", v6CIDR).Debug("v6 allocated CIDR")
-		nCIDRs.v6PodCIDRs = []*net.IPNet{v6CIDR}
+		cidrs.v6PodCIDRs = []*net.IPNet{v6CIDR}
+
+		revertStack.Push(revertFunc)
 	}
 
-	if nCIDRs == nil {
+	if cidrs.v4PodCIDRs == nil && cidrs.v6PodCIDRs == nil {
 		return nil, false, ErrNoAllocators{
 			name: nodeName,
 			v4:   getCIDRAllocatorsInfo(n.v4CIDRAllocators, v4AllocatorType),
 			v6:   getCIDRAllocatorsInfo(n.v6CIDRAllocators, v6AllocatorType),
 		}
 	}
 
-	n.nodes[nodeName] = nCIDRs
-
-	return nCIDRs, true, nil
+	n.nodes[nodeName] = &cidrs
 
+	return &cidrs, true, nil
 }
 
 func getCIDRAllocatorsInfo(cidrAllocators []CIDRAllocator, netTypes string) string {