[S390] vdso: kernel parameter syntax

The syntax of the vdso kernel parameter is documented as vdso=[on|off].
The implementation uses vdso=[0|1], an invalid parameter string disables
the vdso support. Fix the mismatch by adding vdso=[on|off] as additional
parameter syntax.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>