mm: Fix zone->present_pages underflow

If offlined_pages is greater than
zone->present_pages, underflow will occur.

This change will set zone->present_pages to 0 if
offlined_pages is greater.

Change-Id: I728e90c60fb7fc391de7b9c4828ab264ca38653b
Signed-off-by: Jack Cheung <jackc@codeaurora.org>
1 file changed