video: fbdev: core: Change return type to vm_fault_t

Use new return type vm_fault_t for fault handler
and page_mkwrite handler.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Jaya Kumar <jayakumar.lkml@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>