sctp: ensure ep is not destroyed before doing the dump

sctp: ensure ep is not destroyed before doing the dump

Now before dumping a sock in sctp_diag, it only holds the sock while
the ep may be already destroyed. It can cause a use-after-free panic
when accessing ep->asocs.

This patch is to set sctp_sk(sk)->ep NULL in sctp_endpoint_destroy,
and check if this ep is already destroyed before dumping this ep.

Suggested-by: Marcelo Ricardo Leitner <[email protected]>
Signed-off-by: Xin Long <[email protected]>
Acked-by: Neil Horman <[email protected]>
Signed-off-by: David S. Miller <[email protected]>