fix(dhcp): avoid infinite loop building IPv6 reverse zones

getzonesfornet() derives the number of reverse-zone nibbles as
$nibbs = $maskbits / 4, then decrements it once per hex nibble of the network
prefix. For a sub-nibble (not 4-bit-aligned) IPv6 mask $nibbs can go negative
before the padding loop, and `while ($nibbs)` then never terminates: it keeps
decrementing past zero while appending "0." to $rev, spinning forever and
growing the string until the process is killed.

Make the padding loop `while ($nibbs > 0)` so it can never run away, and
return early only when $nibbs is genuinely negative. $nibbs == 0 is the normal
nibble-aligned case (e.g. a /64) and must still emit its reverse zone.

Recovered from the unmerged lenovobuild branch (original 0e070cd2). The
original guarded with `$nibbs < 1`, which also dropped the valid $nibbs == 0
case and left standard /64 subnets with no reverse zone; corrected to
`$nibbs < 0` after lab validation on a real provisioning cluster.

Co-authored-by: Jarrod Johnson <10814490+jjohnson42@users.noreply.github.com>

xCAT-server/lib/xcat/plugins/dhcp.pm

@@ -3629,7 +3629,13 @@ sub getzonesfornet {
             $rev .= $_ . ".";
             $nibbs--;
         }
-        while ($nibbs) {
+        if ($nibbs < 0) {
+            # a sub-nibble (not 4-bit-aligned) IPv6 mask leaves $nibbs negative;
+            # bail rather than spin forever in the loop below. $nibbs == 0 is the
+            # normal nibble-aligned case (e.g. a /64) and must still emit a zone.
+            return ();
+        }
+        while ($nibbs > 0) {
             $rev .= "0.";
             $nibbs--;
         }