Silence a gcc warning about no return from a function that handles every

possible enum in a switch statement.  I verified that this emits nothing
as expected on clang.  radix relies on constant propagation to eliminate
any branching from these access routines.

Reported by:	lwhsu/tinderbox

sys/vm/vm_radix.c

@@ -208,6 +208,8 @@ vm_radix_node_load(smrnode_t *p, enum vm_radix_access access)
 	case SMR:
 		return (smr_entered_load(p, vm_radix_smr));
 	}
+	/* This is unreachable, silence gcc. */
+	panic("vm_radix_node_get: Unknown access type");
 }
 
 static __inline void