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>

security/keys/process_keys.c

@@ -465,7 +465,7 @@ key_ref_t search_cred_keyrings_rcu(struct keyring_search_context *ctx)
 		case -EAGAIN: /* no key */
 			if (ret)
 				break;
-			/* fall through */
+			fallthrough;
 		case -ENOKEY: /* negative key */
 			ret = key_ref;
 			break;
@@ -487,7 +487,7 @@ key_ref_t search_cred_keyrings_rcu(struct keyring_search_context *ctx)
 		case -EAGAIN: /* no key */
 			if (ret)
 				break;
-			/* fall through */
+			fallthrough;
 		case -ENOKEY: /* negative key */
 			ret = key_ref;
 			break;
@@ -509,7 +509,7 @@ key_ref_t search_cred_keyrings_rcu(struct keyring_search_context *ctx)
 		case -EAGAIN: /* no key */
 			if (ret)
 				break;
-			/* fall through */
+			fallthrough;
 		case -ENOKEY: /* negative key */
 			ret = key_ref;
 			break;

security/keys/request_key.c

@@ -295,26 +295,26 @@ static int construct_get_dest_keyring(struct key **_dest_keyring)
 				}
 			}
 
-			/* fall through */
+			fallthrough;
 		case KEY_REQKEY_DEFL_THREAD_KEYRING:
 			dest_keyring = key_get(cred->thread_keyring);
 			if (dest_keyring)
 				break;
 
-			/* fall through */
+			fallthrough;
 		case KEY_REQKEY_DEFL_PROCESS_KEYRING:
 			dest_keyring = key_get(cred->process_keyring);
 			if (dest_keyring)
 				break;
 
-			/* fall through */
+			fallthrough;
 		case KEY_REQKEY_DEFL_SESSION_KEYRING:
 			dest_keyring = key_get(cred->session_keyring);
 
 			if (dest_keyring)
 				break;
 
-			/* fall through */
+			fallthrough;
 		case KEY_REQKEY_DEFL_USER_SESSION_KEYRING:
 			ret = look_up_user_keyrings(NULL, &dest_keyring);
 			if (ret < 0)