srcu: Make Classic and Tree SRCU announce themselves at bootup

Currently, the only way to tell whether a given kernel is running
Classic, Tiny, or Tree SRCU is to look at the .config file, which
can easily be lost or associated with the wrong kernel.  This commit
therefore has Classic and Tree SRCU identify themselves at boot time.

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