libct: close child fds on prepareCgroupFD error

The (*setns).start is supposed to close child fds once the child has
started, or upon an error. Commit 5af4dd4e6 added a bug -- child fds
are not closed if prepareCgroupFD fails.

Fix by adding a missing call to closeChild.

I'm not sure how to write a good test case for it. Found when working
on PR 4928 (and tested in there).

Fixes: 5af4dd4e6
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/process_linux.go

@@ -357,6 +357,7 @@ func (p *setnsProcess) start() (retErr error) {
 
 	fd, err := p.prepareCgroupFD()
 	if err != nil {
+		p.comm.closeChild()
 		return err
 	}