BUG/MINOR: quic: fix CRYPTO payload size calcul for encoding

BUG/MINOR: quic: fix CRYPTO payload size calcul for encoding

Function max_stream_data_size() is used to determine the payload length
of a CRYPTO frame. It takes into account that the CRYPTO length field is
a variable length integer.

Implemented calcul was incorrect as it reserved too much space as a
frame header. This error is mostly due because max_stream_data_size()
reuses max_available_room() which also reserve space for a variable
length integer. This results in CRYPTO frames shorter of 1 to 2 bytes
than the maximum achievable value, which produces in the end datagram
shorter than the MTU.

Fix max_stream_data_size() implementation. It is now merely a wrapper on
max_available_room(). This ensures that CRYPTO frame encoding is now
properly optimized to use the MTU available.

This should be backported up to 2.6.