MINOR: quic: do not proceed to accept for closing conn

MINOR: quic: do not proceed to accept for closing conn

Each quic_conn is inserted in an accept queue to allocate the upper
layers. This is done through a listener tasklet in
quic_sock_accept_conn().

This patch interrupts the accept process for a quic_conn in
closing/draining state. Indeed, this connection will soon be closed so
it's unnecessary to allocate a complete stack for it.

This patch will become necessary when thread migration is implemented.
Indeed, it won't be allowed to proceed to thread migration for a closing
quic_conn.

This should be backported up to 2.7 after a period of observation.