MINOR: quic: Amplification limit handling sanitization.

MINOR: quic: Amplification limit handling sanitization.

Add a BUG_ON() to quic_peer_validated_addr() to check the amplification limit
is respected when it return false(0), i.e. when the connection is not validated.

Implement quic_may_send_bytes() which returns the number of bytes which may be
sent when the connection has not already been validated and call this functions
at several places when this is the case (after having called
quic_peer_validated_addr()).

Furthermore, this patch improves the code maintainability. Some patches to
come will have to rename ->[rt]x.bytes quic_conn struct members.