BACKPORT: kernel: cgroup: add poll file operation

Cgroup has a standardized poll/notification mechanism for waking all
pollers on all fds when a filesystem node changes.  To allow polling for
custom events, add a .poll callback that can override the default.

This is in preparation for pollable cgroup pressure files which have
per-fd trigger configurations.

Link: http://lkml.kernel.org/r/20190124211518.244221-3-surenb@google.com


Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

(cherry picked from commit: dc50537bdd1a0804fa2cbc990565ee9a944e66fa)

Conflicts:
        include/linux/cgroup-defs.h
        kernel/cgroup.c

(1. replaced __poll_t with unsigned int)

Bug: 127712811
Test: lmkd in PSI mode
Change-Id: I21aff1d9d31e3d4b45e257aa4d299405a2ce6de3
Signed-off-by: Suren Baghdasaryan <surenb@google.com>