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 warning:

  security/keys/process_keys.c:380:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
  security/keys/process_keys.c:404:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
  security/keys/process_keys.c:424:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: James Morris <james.morris@microsoft.com>

security/keys/process_keys.c

@@ -379,6 +379,7 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
 		case -EAGAIN: /* no key */
 			if (ret)
 				break;
+			/* fall through */
 		case -ENOKEY: /* negative key */
 			ret = key_ref;
 			break;
@@ -403,6 +404,7 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
 		case -EAGAIN: /* no key */
 			if (ret)
 				break;
+			/* fall through */
 		case -ENOKEY: /* negative key */
 			ret = key_ref;
 			break;
@@ -423,6 +425,7 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
 		case -EAGAIN: /* no key */
 			if (ret)
 				break;
+			/* fall through */
 		case -ENOKEY: /* negative key */
 			ret = key_ref;
 			break;