net/x25: fix nonblocking connect

This patch fixes 2 issues in x25_connect():

1. It makes absolutely no sense to reset the neighbour and the
connection state after a (successful) nonblocking call of x25_connect.
This prevents any connection from being established, since the response
(call accept) cannot be processed.

2. Any further calls to x25_connect() while a call is pending should
simply return, instead of creating new Call Request (on different
logical channels).

This patch should also fix the "KASAN: null-ptr-deref Write in
x25_connect" and "BUG: unable to handle kernel NULL pointer dereference
in x25_connect" bugs reported by syzbot.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Reported-by:  <syzbot+429c200ffc8772bfe070@syzkaller.appspotmail.com>
Reported-by:  <syzbot+eec0c87f31a7c3b66f7b@syzkaller.appspotmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>