sctp: remove the NETIF_F_SG flag before calling skb_segment

It makes more sense to clear NETIF_F_SG instead of set it when
calling skb_segment() in sctp_gso_segment(), since SCTP GSO is
using head_skb's fraglist, of which all frags are linear skbs.

This will make SCTP GSO code more understandable.

Suggested-by: Alexander Duyck <alexander.duyck@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>