mount: handle tracefs more gracefully

See the comment for details, but basically tracefs is automounted by the
kernel, so we can just mount debugfs with MS_REC and get the right result.

v2: rebase on criu-dev
v3: don't use a new fstype->flags, just always set MS_REC in debugfs'
    ->parse

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>