mm: process_reclaim: consider compound pages

Avoid passing tail pages to isolate_lru_page. In the case
process reclaim, since only lru pages are considered, this
is just to avoid a warning from isolate_lru_page.

Change-Id: I1f54dcec15f8c2d5ba16738657e79d2793d36c77
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>