s390: vdso_fault return type

Use new return type vm_fault_t for fault handler vdso_fault.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>