mm/page_owner: clamp read count to PAGE_SIZE

mm/page_owner: clamp read count to PAGE_SIZE

The (root-only) page owner read might allocate a large size of memory with
a large read count.  Allocation fails can easily occur when doing high
order allocations.

Clamp buffer size to PAGE_SIZE to avoid arbitrary size allocation
and avoid allocation fails due to high order allocation.

[[email protected]: use min_t()]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Miles Chen <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>