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 <[email protected]>
diff --git a/crypto/drbg.c b/crypto/drbg.c
index e99fe34..3132967 100644
--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1521,7 +1521,7 @@ static int drbg_prepare_hrng(struct drbg_state *drbg)
 
 	case -EALREADY:
 		err = 0;
-		/* fall through */
+		fallthrough;
 
 	default:
 		drbg->random_ready.func = NULL;