BUG/MEDIUM: dict: hold lock while decrementing refcount in dict_entry_unref

BUG/MEDIUM: dict: hold lock while decrementing refcount in dict_entry_unref

In dict_entry_unref(), the write lock on d->rwlock was only acquired after
decrementing the refcount. However, between the decrement and the lock,
another thread could increment it by calling dict_insert(). That could lead
to a UAF.

To fix the issue, the call to HA_ATOMIC_SUB_FETCH is moved inside the write
lock.

This patch must be backported to all stable versions.