srcu: Document srcu_flip() memory-barrier D relation to SRCU-fast

srcu: Document srcu_flip() memory-barrier D relation to SRCU-fast

The smp_mb() memory barrier at the end of srcu_flip() has a comment,
but that comment does not make it clear that this memory barrier is an
optimization, as opposed to being needed for correctness.  This commit
therefore adds this information and points out that it is omitted
for SRCU-fast, where a much heavier weight synchronize_srcu() would
be required.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: <bpf@vger.kernel.org>