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/lib/assoc_array.c b/lib/assoc_array.c
index 6f4bcf5..04c9879 100644
--- a/lib/assoc_array.c
+++ b/lib/assoc_array.c
@@ -1113,7 +1113,7 @@
 						index_key))
 				goto found_leaf;
 		}
-		/* fall through */
+		fallthrough;
 	case assoc_array_walk_tree_empty:
 	case assoc_array_walk_found_wrong_shortcut:
 	default: