migration/postcopy: Avoid clearing dirty bitmap for postcopy too

migration/postcopy: Avoid clearing dirty bitmap for postcopy too

This is a follow up on the other commit "migration/ram: avoid to do log
clear in the last round" but for postcopy.

https://lore.kernel.org/r/20250514115827.3216082-1-yanfei.xu@bytedance.com

I can observe more than 10% reduction of average page fault latency during
postcopy phase with this optimization:

  Before: 268.00us (+-1.87%)
  After:  232.67us (+-2.01%)

The test was done with a 16GB VM with 80 vCPUs, running a workload that
busy random writes to 13GB memory.

Cc: Yanfei Xu <yanfei.xu@bytedance.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250613140801.474264-12-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>