mm: process_reclaim: pass pid struct instead of tgid

Pass pid struct instead of tgid to the reclaim notifier so that
notifier users need not worry on pid reuse issues.

Change-Id: Ifd11937da75547c3b0ddecc19ecad1e488a86962
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>