mm: fix balloon_page_movable() page->flags check

mm: fix balloon_page_movable() page->flags check

Fix the following crash by fixing and enhancing the way page->flags are
tested to identify a ballooned page.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0

The NULL pointer deref was taking place because balloon_page_movable()
page->flags tests were incomplete and we ended up inadvertently poking at
private pages.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Reported-by: Sasha Levin <levinsasha928@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>