rcu: trace fix possible mem-leak

In the initialization of the RCU trace module, if
rcupreempt_debugfs_init() fails, we never free the the trace buffer.

This patch frees the trace buffer in case the debugfs fails.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Reviewed-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>