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

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>