rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast

rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast

This commit saves more than 500 lines of RCU code by re-implementing
RCU Tasks Trace in terms of SRCU-fast.  Follow-up work will remove
more code that does not cause problems by its presence, but that is no
longer required.

This variant places smp_mb() in rcu_read_{,un}lock_trace(), and in the
same place that srcu_read_{,un}lock() would put them. These smp_mb()
calls will be removed on common-case architectures in a later commit.
In the meantime, it serves to enforce ordering between the underlying
srcu_read_{,un}lock_fast() markers and the intervening critical section,
even on architectures that permit attaching tracepoints on regions of
code not watched by RCU.  Such architectures defeat SRCU-fast's use of
implicit single-instruction, interrupts-disabled, and atomic-operation
RCU read-side critical sections, which have no effect when RCU is not
watching.  The aforementioned later commit will insert these smp_mb()
calls only on architectures that have not used noinstr to prevent
attaching tracepoints to code where RCU is not watching.

[ paulmck: Apply kernel test robot, Boqun Feng, and Zqiang feedback. ]
[ paulmck: Split out Tiny SRCU fixes per Andrii Nakryiko feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: kernel test robot <oliver.sang@intel.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: bpf@vger.kernel.org
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>