RCU: make treercu be default

Impact: switch default config from CLASSIC_RCU to TREE_RCU

Given that I have not gotten any complaints or bug reports on treercu
recently, this patch makes it be the default.  There are a number of
other defconfig files that explicitly call out CLASSIC_RCU, but which
have comment headers saying not to edit them.  Probably holdovers from
one of the flavors of "make config", but...

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: akpm@linux-foundation.org
Cc: dipankar@in.ibm.com
Cc: niv@us.ibm.com
Cc: manfred@colorfullife.com
Cc: peterz@infradead.org
LKML-Reference: <20090403040625.GA9473@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>