BUG/MEDIUM: quic: prevent EMSGSIZE with GSO for larger bufsize

BUG/MEDIUM: quic: prevent EMSGSIZE with GSO for larger bufsize

A UDP datagram cannot be greater than 65535 bytes, as UDP length header
field is encoded on 2 bytes. As such, sendmsg() will reject a bigger
input with error EMSGSIZE. By default, this does not cause any issue as
QUIC datagrams are limited to 1.252 bytes and sent individually.

However, with GSO support, value bigger than 1.252 bytes are specified
on sendmsg(). If using a bufsize equal to or greater than 65535, syscall
could reject the input buffer with EMSGSIZE. As this value is not
expected, the connection is immediately closed by haproxy and the
transfer is interrupted.

This bug can easily reproduced by requesting a large object on loopback
interface and using a bufsize of 65535 bytes. In fact, the limit is
slightly less than 65535, as extra room is also needed for IP + UDP
headers.

Fix this by reducing the count of datagrams encoded in a single GSO
invokation via qc_prep_pkts(). Previously, it was set to 64 as specified
by man 7 udp. However, with 1252 datagrams, this is still too many.
Reduce it to a value of 52. Input to sendmsg will thus be restricted to
at most 65.104 bytes if last datagram is full.

If there is still data available for encoding in qc_prep_pkts(), they
will be written in a separate batch of datagrams. qc_send_ppkts() will
then loop over the whole QUIC Tx buffer and call sendmsg() for each
series of at most 52 datagrams.

This does not need to be backported.