srcutree: Explicitly note DEFINE_SRCU() needs for srcu_barrier()

In the core kernel, an srcu_struct structure created by DEFINE_SRCU()
or friends lives as long as the kernel does, so there are no particular
requirements surrounding the end of that structure's life.  In contrast,
when DEFINE_SRCU() and friends are used within a module, the corresponding
srcu_struct structures' lifetimes end when that module exits.  This in
turn means that if such a structure was passed to call_srcu(), then
srcu_barrier() must be invoked after the last call_srcu() invocation
but before the module exits.

This commit therefore adds a comment stating this.

Reported-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>