rcu: undeclared variable used in documentation

The RCU documentation uses an fp variable which is not declared in the code
snippets. Use the new_fp variable instead.

Signed-Off-By: Baruch Even <baruch@ev-en.org>
Acked-by:  <paulmck@us.ibm.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>