mm: put pages into alloc state after offline

When the pages are taken out of buddy list during memory offline,
put the pages into alloc state which the free_pages expects it to
be when onlining these pages.

Change-Id: I04b735a8089a9c4a91131fe8aa95625ce8904c57
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>