migration: Fix low possibility downtime violation

When QEMU queried the estimated version of pending data and thinks it's
ready to converge, it'll send another accurate query to make sure of it.
It is needed to make sure we collect the latest reports and that equation
still holds true.

However we missed one tiny little difference here on "<" v.s. "<=" when
comparing pending_size (A) to threshold_size (B)..

QEMU src only re-query if A<B, but will kickoff switchover if A<=B.

I think it means it is possible to happen if A (as an estimate only so far)
accidentally equals to B, then re-query won't happen and switchover will
proceed without considering new dirtied data.

It turns out it was an accident in my commit 7aaa1fc072 when refactoring
the code around.  Fix this by using the same equation in both places.

Fixes: 7aaa1fc072 ("migration: Rewrite the migration complete detect logic")
Cc: qemu-stable@nongnu.org
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20260421202110.306051-3-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/migration.c

@@ -3258,7 +3258,7 @@ static MigIterateState migration_iteration_run(MigrationState *s)
          * postcopy started, so ESTIMATE should always match with EXACT
          * during postcopy phase.
          */
-        if (pending_size < s->threshold_size) {
+        if (pending_size <= s->threshold_size) {
             qemu_savevm_state_pending_exact(&must_precopy, &can_postcopy);
             pending_size = must_precopy + can_postcopy;
             trace_migrate_pending_exact(pending_size, must_precopy,