net-sctp: replace some sock_net(sk) with just 'net'

It already existed in part of the function, but move it
to a higher level and use it consistently throughout.

Safe since sk is never written to.

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>