migration/multifd: Fix clearing of mapped-ram zero pages

migration/multifd: Fix clearing of mapped-ram zero pages

When the zero page detection is done in the multifd threads, we need
to iterate the second part of the pages->offset array and clear the
file bitmap for each zero page. The piece of code we merged to do that
is wrong.

The reason this has passed all the tests is because the bitmap is
initialized with zeroes already, so clearing the bits only really has
an effect during live migration and when a data page goes from having
data to no data.

Fixes: 303e6f54f9 ("migration/multifd: Implement zero page transmission on the multifd thread.")
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240321201242.6009-1-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>