fs: ubifs: Adding new return type vm_fault_t

Use new return type vm_fault_t for page_mkwrite handler.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Richard Weinberger <richard@nod.at>