rcu: Move rcutiny.h to new empty/true/false-function style

This commit saves a few lines in include/linux/rcutiny.h by moving
to single-line definitions for empty functions, instead of the old
style where the two curly braces each get their own line.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>