fs: nfs: Adding new return type vm_fault_t

Use new return type vm_fault_t for 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.

see commit 1c8f4220 ("mm: change return type to
vm_fault_t") for reference.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>