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

    Reported-by: Ben Nagy <ben@iagu.net>
    Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
    Tested-by: Ben Nagy <ben@iagu.net>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Avi Kivity <avi@redhat.com>
    Cc: John Stultz <johnstul@us.ibm.com>
    Cc: Richard Cochran <richard.cochran@omicron.at>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

    Conflicts:

    	include/linux/posix-timers.h