resolve: mdns: calculate required packet size to store questions and authorities
    
    Otherwise, if we have many cached entries or pending transactions with
    TYPE_ANY, then dns_transaction_make_packet_mdns() fails with -EMSGSIZE.
    
    This also fixes use-after-free.
    
    Fixes #23894.
    
    (cherry picked from commit 325513bc776c739a814996cc5c483235ca92be86)