posix-timers: RCU conversion
Ben Nagy reported a scalability problem with KVM/QEMU that hit very hard
    a single spinlock (idr_lock) in posix-timers code, on its 48 core
    machine.

    Even on a 16 cpu machine (2x4x2), a single test can show 98% of cpu time
    used in ticket_spin_lock, from lock_timer

    Ref: http://www.spinics.net/lists/kvm/msg51526.html

    Switching to RCU is quite easy, IDR being already RCU ready. idr_lock
    should be locked only for an insert/delete, not a lookup.

    Benchmark on a 2x4x2 machine, 16 processes calling timer_gettime().

    Before :

    real    1m18.669s
    user    0m1.346s
    sys     1m17.180s

    After :

    real    0m3.296s
    user    0m1.366s
    sys     0m1.926s