At present, a return from vm_fault() of KERN_OUT_OF_BOUNDS can signal a pager failure or can indicate that VM_FAULT_NOFILL was successful in avoiding a fill. As some callers (e.g., the prototype for CHERI capability revocation) may want to distinguish these cases, use a different return code. KERN_NOT_RECEIVER is not otherwise an expected return from vm_fault(), so go ahead and use it (again) as a special case.
The sole existing in-tree user of VM_FAULT_NOFILL seems to be sys/kern/kern_exec.c:/^core_output and that path does not discriminate on return codes beyond KERN_SUCCESS vs. anything else. This change thus does not alter its behaviour.