sctp: remove the unessessary hold for idev in sctp_v6_err

Same as in tcp_v6_err() and __udp6_lib_err(), there's no need to
hold idev in sctp_v6_err(), so just call __in6_dev_get() instead.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>