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/mpls/af_mpls.c b/net/mpls/af_mpls.c
index 6fdd0c9..f2868a8 100644
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c
@@ -1516,7 +1516,7 @@ static void mpls_ifdown(struct net_device *dev, int event)
 			case NETDEV_DOWN:
 			case NETDEV_UNREGISTER:
 				nh_flags |= RTNH_F_DEAD;
-				/* fall through */
+				fallthrough;
 			case NETDEV_CHANGE:
 				nh_flags |= RTNH_F_LINKDOWN;
 				break;