rcu: move rcupdate.h to the new true/false-function style

This commit saves a few lines in include/linux/rcupdate.h by moving
to single-line definitions for functions that just return either true
or false, instead of the old style where the two curly braces each get
their own line.

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