lib: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding multiple break statements instead of
letting the code fall through to the next case, and by replacing a
number of /* fall through */ comments with the new pseudo-keyword
macro fallthrough.

Notice that Clang doesn't recognize /* Fall through */ comments as
implicit fall-through markings.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
diff --git a/lib/nlattr.c b/lib/nlattr.c
index 74019c8..1937235 100644
--- a/lib/nlattr.c
+++ b/lib/nlattr.c
@@ -432,7 +432,7 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 			err = -EINVAL;
 			goto out_err;
 		}
-		/* fall through */
+		fallthrough;
 
 	case NLA_STRING:
 		if (attrlen < 1)