rcu: Use pr_fmt to prefix "rcu: " to logging output

This commit also adjusts some whitespace while in the area.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Revert string-breaking %s as requested by Andy Shevchenko. ]