Merge branch 'rcu/srcu' into next

_ Properly handle SRCU readers within IRQ disabled sections in tiny SRCU

- Preparation to reimplement RCU Tasks Trace on top of SRCU fast:

    - Introduce API to expedite a grace period and test it through
      rcutorture.

    - Split srcu-fast in two flavours: SRCU-fast and SRCU-fast-updown.
      Both are still targeted toward faster readers (without full
      barriers on LOCK and UNLOCK) at the expense of heavier write side
      (using full RCU grace period ordering instead of simply full
      ordering) as compared to "traditional" non-fast SRCU. But those
      srcu-fast flavours are going to be optimized in two different
      ways:

         - SRCU-fast will become the reimplementation basis for
           RCU-TASK-TRACE for consolidation. Since RCU-TASK-TRACE must
           be NMI safe, SRCU-fast must be as well.

         - SRCU-fast-updown will be needed for uretprobes code in order
           to get rid of the read-side memory barriers while still
           allowing entering the reader at task level while exiting it
           in a timer handler. It is considered semaphore-like in that
           it can have different owners between LOCK and UNLOCK.
           However it is not NMI-safe.

      The actual optimizations are work in progress for the next cycle.
      Only the new interfaces are added for now, along with related
      torture and scalability test code.

- Create/document/debug/torture new proper initializers for RCU fast:
   DEFINE_SRCU_FAST() and init_srcu_struct_fast()

   This allows for using right away the proper ordering on the write
   side (either full ordering or full RCU grace period ordering) without
   waiting for the read side to tell which to use. Also this optimizes
   the read side altogether with moving flavour debug checks to debug
   config and with removing a costly RmW operation on their first call.

- Make some diagnostic functions tracing safe.