security: keys: annotate implicit fall through

There is a plan to build the kernel with -Wimplicit-fallthrough and
this place in the code produced a warning (W=1).

This commit remove the following warning:

  security/keys/keyring.c:248:10: 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/keyring.c

@@ -246,6 +246,7 @@ static unsigned long keyring_get_key_chunk(const void *data, int level)
 				    (ASSOC_ARRAY_KEY_CHUNK_SIZE - 8));
 		n--;
 		offset = 1;
+		/* fall through */
 	default:
 		offset += sizeof(chunk) - 1;
 		offset += (level - 3) * sizeof(chunk);