btrfs: Don't fail if we meet non-subvolume mountpoint

If mount point is not a subvolume we need more complex logic
to figure out which subvolumes it might have. It'll be addressed
later, at moment simply don't fail in such case: for worst
scenarion we will fail later in CRIU code when devs are not
matching, in best case if application is not using notify,
sockets or deleted files -- we should continue without errors.

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