fs: kernfs: Adding new return type vm_fault_t

Use new return type vm_fault_t for page_mkwrite and
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.

Reference id -> 1c8f4220 ("mm: change return type to
vm_fault_t")

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>