BugLink: http://bugs.launchpad.net/bugs/1744873

commit 11d49e9d upstream.

we are advancing sg as we go, so the pages we need to drop in
case of error are *before* the current sg.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>