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/net/netdevsim/bus.c

@@ -193,7 +193,7 @@ new_device_store(struct bus_type *bus, const char *buf, size_t count)
 	switch (err) {
 	case 1:
 		port_count = 1;
-		/* fall through */
+		fallthrough;
 	case 2:
 		if (id > INT_MAX) {
 			pr_err("Value of \"id\" is too big.\n");

drivers/net/netdevsim/fib.c

@@ -760,14 +760,14 @@ static int nsim_fib_event_nb(struct notifier_block *nb, unsigned long event,
 	spin_lock_bh(&data->fib_lock);
 
 	switch (event) {
-	case FIB_EVENT_RULE_ADD: /* fall through */
+	case FIB_EVENT_RULE_ADD:
 	case FIB_EVENT_RULE_DEL:
 		err = nsim_fib_rule_event(data, info,
 					  event == FIB_EVENT_RULE_ADD);
 		break;
 
-	case FIB_EVENT_ENTRY_REPLACE:  /* fall through */
-	case FIB_EVENT_ENTRY_APPEND:  /* fall through */
+	case FIB_EVENT_ENTRY_REPLACE:
+	case FIB_EVENT_ENTRY_APPEND:
 	case FIB_EVENT_ENTRY_DEL:
 		err = nsim_fib_event(data, info, event);
 		break;