mm: process_reclaim: do not iterate over stale task structs

swap_fn iterates through the threads of selected tasks after
a rcu_read_unlock which is wrong. But we can't extend the
rcu_read_lock since it will result in severe performance
issues. So better avoid iterating over the threads. Just
lock the group leader and use it further.

Change-Id: I36269b1b6619315f33f6f3b49ec73571a66796f2
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>