orangefs: Adding new return type 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 the following
commit 1c8f4220 ("mm: change return type to vm_fault_t")

Fixed checkpatch.pl warning.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>