sctp: handle the error returned from sctp_auth_asoc_init_active_key

When it returns an error from sctp_auth_asoc_init_active_key(), the
active_key is actually not updated. The old sh_key will be freeed
while it's still used as active key in asoc. Then an use-after-free
will be triggered when sending patckets, as found by syzbot:

  sctp_auth_shkey_hold+0x22/0xa0 net/sctp/auth.c:112
  sctp_set_owner_w net/sctp/socket.c:132 [inline]
  sctp_sendmsg_to_asoc+0xbd5/0x1a20 net/sctp/socket.c:1863
  sctp_sendmsg+0x1053/0x1d50 net/sctp/socket.c:2025
  inet_sendmsg+0x99/0xe0 net/ipv4/af_inet.c:819
  sock_sendmsg_nosec net/socket.c:714 [inline]
  sock_sendmsg+0xcf/0x120 net/socket.c:734

This patch is to fix it by not replacing the sh_key when it returns
errors from sctp_auth_asoc_init_active_key() in sctp_auth_set_key().
For sctp_auth_set_active_key(), old active_key_id will be set back
to asoc->active_key_id when the same thing happens.

Fixes: 58acd100

 ("sctp: update active_key for asoc when old key is being replaced")
Reported-by:  <syzbot+a236dd8e9622ed8954a3@syzkaller.appspotmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>