srcu: Create an SRCU-fast-updown API

srcu: Create an SRCU-fast-updown API

This commit creates an SRCU-fast-updown API, including
DEFINE_SRCU_FAST_UPDOWN(), DEFINE_STATIC_SRCU_FAST_UPDOWN(),
__init_srcu_struct_fast_updown(), init_srcu_struct_fast_updown(),
srcu_read_lock_fast_updown(), srcu_read_unlock_fast_updown(),
__srcu_read_lock_fast_updown(), and __srcu_read_unlock_fast_updown().

These are initially identical to their SRCU-fast counterparts, but both
SRCU-fast and SRCU-fast-updown will be optimized in different directions
by later commits. SRCU-fast will lack any sort of srcu_down_read() and
srcu_up_read() APIs, which will enable extremely efficient NMI safety.
For its part, SRCU-fast-updown will not be NMI safe, which will enable
reasonably efficient implementations of srcu_down_read_fast() and
srcu_up_read_fast().

This API fork happens to meet two different future use cases.

* 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.

This commit also adds rcutorture tests for the new APIs.  This
(annoyingly) needs to be in the same commit for bisectability.  With this
commit, the 0x8 value tests SRCU-fast-updown.  However, most SRCU-fast
testing will be via the RCU Tasks Trace wrappers.

[ paulmck: Apply s/0x8/0x4/ missing change per Boqun Feng feedback. ]
[ paulmck: Apply Akira Yokosawa feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>