mm: process_reclaim: fix shared anon reclaim

Exit as soon as rmap_one is called for the target vma.
No need to unmap page from rest of vmas.

Fixes: 434fe4e6 ("mm: Enhance per process reclaim to consider shared pages")
Change-Id: I583704b7623c4ff57b6acee7458910fab833adcd
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>