selftests/net: Add sk_bind_sendto_listen and sk_connect_zero_addr

This patch adds 2 new tests: sk_bind_sendto_listen and
sk_connect_zero_addr.

The sk_bind_sendto_listen test exercises the path where a socket's
rcv saddr changes after it has been added to the binding tables,
and then a listen() on the socket is invoked. The listen() should
succeed.

The sk_bind_sendto_listen test is copied over from one of syzbot's
tests: https://syzkaller.appspot.com/x/repro.c?x=1673a38df00000

The sk_connect_zero_addr test exercises the path where the socket was
never previously added to the binding tables and it gets assigned a
saddr upon a connect() to address 0.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>