mm: vmscan: lock page on swap error in pageout

A workaround was added ealier to move a page to active
list if swapping to devices like zram fails. But this
can result in try_to_free_swap being called from
shrink_page_list, without a properly locked page.
Lock the page when we indicate to activate a page
in pageout().
Add a check to ensure that error is on swap, and
clear the error flag before moving the page to
active list.

CRs-fixed: 760049
Change-Id: I77a8bbd6ed13efdec943298fe9448412feeac176
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>