security: keys: annotate implicit fall throughs

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1). Fix them up.

This commit remove the following warnings:

security/keys/request_key.c:293:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
security/keys/request_key.c:298:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
security/keys/request_key.c:307:7: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

security/keys/request_key.c

···
 				}
 			}
 
+			/* fall through */
 		case KEY_REQKEY_DEFL_THREAD_KEYRING:
 			dest_keyring = key_get(cred->thread_keyring);
 			if (dest_keyring)
 				break;
 
+			/* fall through */
 		case KEY_REQKEY_DEFL_PROCESS_KEYRING:
 			dest_keyring = key_get(cred->process_keyring);
 			if (dest_keyring)
 				break;
 
+			/* fall through */
 		case KEY_REQKEY_DEFL_SESSION_KEYRING:
 			rcu_read_lock();
 			dest_keyring = key_get(
···
 			if (dest_keyring)
 				break;
 
+			/* fall through */
 		case KEY_REQKEY_DEFL_USER_SESSION_KEYRING:
 			dest_keyring =
 				key_get(cred->user->session_keyring);