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 <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Tested-by: Ben Nagy <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Avi Kivity <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Richard Cochran <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>

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