mm: reduce the time spend by killed tasks in alloc path

There are issues reported where the tasks killed by LMK
holding huge amounts of memory, loops for seconds in the
reclaim path, thus causing OOMs to happen from other
contexts or a panic when oom path finds that there are no
killable tasks. This patch brings back a change in older
kernel versions to avoid reclaim when a fatal signal is
pending. This is more improtant in our case unlike upstream,
as we loop almost forever in reclaim path when there are LMK
killable tasks (see lmk_kill_possible). Another change done
by the patch is to return without sleep in too_many_isolated
case for tasks with fatal signal pending.

Change-Id: Icd2bb7a9602ea6566425f7918e34c218bbed21cb
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>