genwqe: Remove unused variable and rename function

This is a clean-up patch, no functional changes intended.

It removes an unused variable from do_execute_ddcb() and
also renames the function free_user_pages(), prepending
"genwqe" prefix in order to clarify the code.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>