staging/zcache: Fix memory leak while dropping an o2net_sock_container object

Free sc->sc_page when releasing a struct o2net_sock_container
object to avoid memory leaks.

This issue has been fixed by Younger Liu for OCFS2, so I sync
that patch to zcache as we share the same network code base.

Cc: Younger Liu <younger.liu@huawei.com>
Cc: Bob Liu <bob.liu@oracle.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>