mm: Don't use CMA pages for page cache

All layers of the page cache may take extra references to
pages to avoid migration. This is fine for general movable
pages but renders CMA pages useless as they cannot be
allocated for contiguous mode. Always omitting movable
pages completely has poor utilization of memory so attempt
with the given GFP flags first and if a CMA page is allocated,
free it and get a new non-movable page. This does have a minor
performance impact, but ultimately it's a tradeoff between
never using movable pages and taking the extra hit of a
free/realloc.

Change-Id: I4e9d7fc1c65306d6356492fd1a99935aa62ed2ec
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>