migration: Drop unnecessary check in ram's pending_exact()

When the migration frameworks fetches the exact pending sizes, it means
this check:

  remaining_size < s->threshold_size

Must have been done already, actually at migration_iteration_run():

    if (must_precopy <= s->threshold_size) {
        qemu_savevm_state_pending_exact(&must_precopy, &can_postcopy);

That should be after one round of ram_state_pending_estimate().  It makes
the 2nd check meaningless and can be dropped.

To say it in another way, when reaching ->state_pending_exact(), we
unconditionally sync dirty bits for precopy.

Then we can drop migrate_get_current() there too.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240117075848.139045-3-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/ram.c

@@ -3213,21 +3213,20 @@ static void ram_state_pending_estimate(void *opaque, uint64_t *must_precopy,
 static void ram_state_pending_exact(void *opaque, uint64_t *must_precopy,
                                     uint64_t *can_postcopy)
 {
-    MigrationState *s = migrate_get_current();
     RAMState **temp = opaque;
     RAMState *rs = *temp;
+    uint64_t remaining_size;
 
-    uint64_t remaining_size = rs->migration_dirty_pages * TARGET_PAGE_SIZE;
-
-    if (!migration_in_postcopy() && remaining_size < s->threshold_size) {
+    if (!migration_in_postcopy()) {
         bql_lock();
         WITH_RCU_READ_LOCK_GUARD() {
             migration_bitmap_sync_precopy(rs, false);
         }
         bql_unlock();
-        remaining_size = rs->migration_dirty_pages * TARGET_PAGE_SIZE;
     }
 
+    remaining_size = rs->migration_dirty_pages * TARGET_PAGE_SIZE;
+
     if (migrate_postcopy_ram()) {
         /* We can do postcopy, and all the data is postcopiable */
         *can_postcopy += remaining_size;