sctp: change to hold sk after auth shkey is created successfully

Now in sctp_endpoint_init(), it holds the sk then creates auth
shkey. But when the creation fails, it doesn't release the sk,
which causes a sk defcnf leak,

Here to fix it by only holding the sk when auth shkey is created
successfully.

Fixes: a29a5bd4

 ("[SCTP]: Implement SCTP-AUTH initializations.")
Reported-by:  <syzbot+afabda3890cc2f765041@syzkaller.appspotmail.com>
Reported-by:  <syzbot+276ca1c77a19977c0130@syzkaller.appspotmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>