char: mspec: change return type to vm_fault_t

Use new return type vm_fault_t for the fault handler
in struct vm_operations_struct.  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.

This driver failed to handle any error returned by
vm_insert_pfn. Use the new vmf_insert_pfn function
to return the correct value.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>