Revert "sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem"

This reverts commit d59cfc09c32a2ae31f1c3bc2983a0cd79afb3f14.

d59cfc09c32a ("sched, cgroup: replace signal_struct->group_rwsem with
a global percpu_rwsem") and b5ba75b5fc0e ("cgroup: simplify
threadgroup locking") changed how cgroup synchronizes against task
fork and exits so that it uses global percpu_rwsem instead of
per-process rwsem; unfortunately, the write [un]lock paths of
percpu_rwsem always involve synchronize_rcu_expedited() which turned
out to be too expensive.

Improvements for percpu_rwsem are scheduled to be merged in the coming
v4.4-rc1 merge window which alleviates this issue.  For now, revert
the two commits to restore per-process rwsem.  They will be re-applied
for the v4.4-rc1 merge window.

Signed-off-by: Tejun Heo <[email protected]>
Link: http://lkml.kernel.org/g/[email protected]
Reported-by: Christian Borntraeger <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: [email protected] # v4.2+

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