fix memory leak in bio handling

pthread_mutex uses lazy_indirect object to allocate actual object
on a first use. The lazily allocated object is not destroyed unless
pthread_mutex_destroy is called.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>