cgroup_freezer doesn't transition from FREEZING to FROZEN if the
cgroup contains PF_NOFREEZE tasks or tasks sleeping with
PF_FREEZER_SKIP set.

Only kernel tasks can be non-freezable (PF_NOFREEZE) and there's
nothing cgroup_freezer or userland can do about or to it.  It's
pointless to stall the transition for PF_NOFREEZE tasks.

PF_FREEZER_SKIP indicates that the task can be skipped when
determining whether frozen state is reached.  A task with
PF_FREEZER_SKIP is guaranteed to perform try_to_freeze() after it
wakes up and can be considered frozen much like stopped or traced
tasks.  Note that a vfork parent uses PF_FREEZER_SKIP while waiting
for the child.

This updates update_if_frozen() such that it only considers freezable
tasks and treats %true freezer_should_skip() tasks as frozen.

This allows cgroups w/ kthreads and vfork parents successfully reach
FROZEN state.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>