rcu: get rid of obsolete "classic" names in TREE_RCU tracing

The TREE_RCU tracing had obsolete rcuclassic_trace_init() and
rcuclassic_trace_cleanup() function names.  This commit brings them
up to date: rcutree_trace_init() and rcutree_trace_cleanup(),
respectively.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>