sparc/oprofile: Convert timer_stop() to use synchronize_rcu()

Now that synchronize_rcu() waits for preempt-disable regions of code
in addition to explicitly marked RCU read-side critical sections,
synchronize_rcu() can be used in place of synchronize_sched().  This
commit therefore makes that change.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Robert Richter <rric@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: <oprofile-list@lists.sf.net>
Cc: <sparclinux@vger.kernel.org>