rcu: Make RCU's CPU-stall detector be default

The RCU_CPU_STALL_DETECTOR costs almost nothing and has located
some bugs that might otherwise have been difficult to track
down.  Make it be default for the TREE RCU implementations.

The vmlinux size impact is limited (on 64-bit x86 defconfig):

   text	   data	    bss	    dec	    hex	filename
   8440248	1260076	 995588	10695912	 a334e8	vmlinux.before
   8440774	1260060	 995588	10696422	 a336e6	vmlinux.after

+526 bytes - acceptable default cost.

For RAM starved systems, TINY_RCU does not support CPU-stall detection
and is much smaller, but then again it is a uniprocessor...

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Lai Jiangshan <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: <12597846162906-git-send-email->
[ v2: added image size calculations to the changelog ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>