percpu: make access macros universal

Now that per_cpu__ prefix is gone, there's no distinction between
static and dynamic percpu variables.  Make get_cpu_var() take dynamic
percpu variables and ensure that all macros have parentheses around
the parameter evaluation and evaluate the variable parameter only once
such that any expression which evaluates to percpu address can be used
safely.

Signed-off-by: Tejun Heo <tj@kernel.org>