files-reg: Don't fail matching if ghost file is on BTRFS subvolume

In case if ghost file is on BTRFS subvolume we should not fail
but test if there is a match.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>