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>
diff --git a/net/rose/rose_route.c b/net/rose/rose_route.c
index 5277631..6e35703 100644
--- a/net/rose/rose_route.c
+++ b/net/rose/rose_route.c
@@ -343,7 +343,7 @@ static int rose_del_node(struct rose_route_struct *rose_route,
 				case 0:
 					rose_node->neighbour[0] =
 						rose_node->neighbour[1];
-					/* fall through */
+					fallthrough;
 				case 1:
 					rose_node->neighbour[1] =
 						rose_node->neighbour[2];
@@ -505,7 +505,7 @@ void rose_rt_device_down(struct net_device *dev)
 				switch (i) {
 				case 0:
 					t->neighbour[0] = t->neighbour[1];
-					/* fall through */
+					fallthrough;
 				case 1:
 					t->neighbour[1] = t->neighbour[2];
 				case 2: