kernel/relay.c: change return type to vm_fault_t

kernel/relay.c: 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.

commit 1c8f422059ae ("mm: change return type to vm_fault_t")

Link: http://lkml.kernel.org/r/20180510140335.GA25363@jordon-HP-15-Notebook-PC
Signed-off-by: Souptick Joarder <[email protected]>
Reviewed-by: Matthew Wilcox <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Eric Biggers <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>