rcu: Fix sparse warnings

Rename local variable "i" in rcu_init() to avoid conflict with
RCU_INIT_FLAVOR(), restrict the scope of RCU_TREE_NONCORE, and
make __synchronize_srcu() static.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <12635142581560-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>