powerpc/cell/spufs: 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. See commit 1c8f4220 ("mm: change return
type to vm_fault_t").

We are fixing a minor bug, that the error from vm_insert_pfn() was
being ignored and the effect of this is likely to be only felt in OOM
situations.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>