keys: Reuse keyring_index_key::desc_len in lookup_user_key()
When lookup_user_key() checks whether the key is possessed, it should
use the key's existing index_key including the 'desc_len' field, rather
than recomputing the 'desc_len'. This doesn't change the behavior; this
way is just simpler and faster.
Signed-off-by: Eric Biggers <ebiggers@google.com> Signed-off-by: David Howells <dhowells@redhat.com> Reviewed-by: James Morris <jamorris@linux.microsoft.com>