migration: write zero pages when postcopy enabled

migration: write zero pages when postcopy enabled

During multifd migration, zero pages are written if
they are migrated more than once.

This may result in a migration thread hang issue when
multifd and postcopy are enabled together.

When postcopy is enabled, always write zero pages as and
when they are migrated.

Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250512125124.147064-2-ppandit@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>