sctp: hold the inp lock while calling ip6_output
This fixes an issue with handling IPPROTO_IPV6 level socket
options.
Reported by: syzbot+66ede232c3d1271c6226@syzkaller.appspotmail.com
MFC after: 3 days
(cherry picked from commit a12d89332efe0b02307c580a6f3025652374cd47)