MINOR: quic: only use sendmsg() syscall variant

MINOR: quic: only use sendmsg() syscall variant

This patch is the direct followup of the previous one :
  MINOR: quic: remove sendto() usage variant

This finalizes qc_snd_buf() simplification by removing send() syscall
usage for quic-conn owned socket. Syscall invocation is merged in a
single code location to the sendmsg() variant.

The only difference for owned socket is that destination address for
sendmsg() is set to NULL. This usage is documented in man 2 sendmsg as
valid for connected sockets. This allows maximum performance by avoiding
unnecessary lookups on kernel socket address tables.

As the previous patch, no functional change should happen here. However,
it will be simpler to extend qc_snd_buf() for GSO usage.