sctp: do not mark sk dumped when inet_sctp_diag_fill returns err

sctp_diag would not actually dump out sk/asoc if inet_sctp_diag_fill
returns err, in which case it shouldn't mark sk dumped by setting
cb->args[3] as 1 in sctp_sock_dump().

Otherwise, it could cause some asocs to have no parent's sk dumped
in 'ss --sctp'.

So this patch is to not set cb->args[3] when inet_sctp_diag_fill()
returns err in sctp_sock_dump().

Fixes: 8f840e47

 ("sctp: add the sctp_diag.c file")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>