sctp: read sk->sk_bound_dev_if once in sctp_rcv()

sctp_rcv() reads sk->sk_bound_dev_if twice while the socket
is not locked. Another cpu could change this field under us.

Fixes: 0fd9a65a ("[SCTP] Support SO_BINDTODEVICE socket option on incoming packets.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>