cgroup: introduce cgroup_taskset and use it in subsys->can_attach(), cancel_attach() and attach()

Currently, there's no way to pass multiple tasks to cgroup_subsys
methods necessitating the need for separate per-process and per-task
methods.  This patch introduces cgroup_taskset which can be used to
pass multiple tasks and their associated cgroups to cgroup_subsys
methods.

Three methods - can_attach(), cancel_attach() and attach() - are
converted to use cgroup_taskset.  This unifies passed parameters so
that all methods have access to all information.  Conversions in this
patchset are identical and don't introduce any behavior change.

-v2: documentation updated as per Paul Menage's suggestion.

Signed-off-by: Tejun Heo <[email protected]>
Reviewed-by: KAMEZAWA Hiroyuki <[email protected]>
Reviewed-by: Frederic Weisbecker <[email protected]>
Acked-by: Paul Menage <[email protected]>
Acked-by: Li Zefan <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Daisuke Nishimura <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: James Morris <[email protected]>

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