s390/vdso: reuse kstrtobool for option value parsing

"vdso" option setup already recognises integer and textual values. Yet
kstrtobool is a more common way to parse boolean values, reuse it to
unify option value parsing behavior and simplify code a bit.

While at it, __setup value parsing callbacks are expected to return
1 when an option is recognized, and returning any other value won't
trigger any error message currently, so simply return 1.

Also don't change default vdso_enabled value of 1 when "vdso" option
value is invalid.

Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>