ke/smr: allow invalid sequences in deferred_advance_commit

This was buggy when the race was lost and it ended up trying to commit a
deferred advance of 0, now explicitly make the magazine's sequence invalid and
do nothing in deferred_advance_commit, avoiding an integer underflow.

src/kern/smr.zig

@@ -274,6 +274,8 @@ pub fn deferred_advance(dom: *Domain) Sequence {
 /// Actually advance the global clock to a value previously returned by
 /// deferred_advance().
 pub fn deferred_advance_commit(dom: *Domain, seq: Sequence) void {
+    if (seq == seq_invalid) return;
+
     _ = dom.clock.write_seq.cmpxchgStrong(
         seq - seq_incr,
         seq,

src/mm/zone.zig

@@ -792,6 +792,10 @@ pub const Zone = struct {
                         // We lost the race. If the magazine we got was fully filled,
                         // then put it in the depot. Otherwise, dispose of it.
                         if (cnt == magazine_size.load()) {
+                            // Ensure this magazine isn't counted as a freed one,
+                            // it is instantly reclaimable.
+                            nm.seq = ke.smr.seq_invalid;
+
                             Depot.free(nm, &cpu.depot.full_mags, self.reuse_policy);
                             continue;
                         }