char: agp: Change return type to vm_fault_t

Use new return type vm_fault_t for fault handler. For now,
this is just documenting that the function returns a
VM_FAULT value rather than an errno. Once all instances are
converted, vm_fault_t will become a distinct type.

Ref-> commit 1c8f4220 ("mm: change return type to
vm_fault_t") was added in 4.17-rc1 to introduce the new
typedef vm_fault_t. Currently we are making change to all
drivers to return vm_fault_t for page fault handlers. As
part of that char/agp driver is also getting changed to
return vm_fault_t type from fault handler.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>