treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/pci/setup-bus.c

@@ -1253,7 +1253,7 @@ void __pci_bus_size_bridges(struct pci_bus *bus, struct list_head *realloc_head)
 			additional_mmio_size = pci_hotplug_mmio_size;
 			additional_mmio_pref_size = pci_hotplug_mmio_pref_size;
 		}
-		/* Fall through */
+		fallthrough;
 	default:
 		pbus_size_io(bus, realloc_head ? 0 : additional_io_size,
 			     additional_io_size, realloc_head);