srcu: Introduce srcu_read_{un,}lock_notrace()

Joel Fernandes is using SRCU to protect from-idle tracepoints, which
requires notrace variants of srcu_read_lock() and srcu_read_unlock()
in order to avoid problems with tracepoints in lockdep.  This commit
therefore adds srcu_read_lock_notrace() and srcu_read_unlock_notrace().

[1] http://lkml.kernel.org/r/20180427042656.190746-1-joelaf@google.com



Reported-by: Joel Fernandes <joelaf@google.com>
Intermittently-reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>