sctp: improve handling of socket shutdown for reading

If a socket is marked as cannot read anymore, drop chunks which
should be added to a control element in the receive queue.
This is consistent with dropping control elements instead of
adding them in the same situation.

Reported by: syzbot+291f6581cecb77097b16@syzkaller.appspotmail.com

(cherry picked from commit 847fa61fad5ef118dc0591d876bf9449200aa818)