mm: process reclaim: skip locked vmas

Skip VMA_LOCKED vmas from being reclaimed. Not doing so results
in pages in unevictable lrus to be passed for reclaim which can
cause issues. In one such reported problem, the trylock_page in
shrink_page_list fails and reclaim path decides to keep the page
and hits VM_BUG_ON_PAGE since PageUnevictable is set.

Change-Id: I58947faeb5226cf8578541d7fdcd474a24f8c40f
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>