mnt: Allow for external mounts to be fsroot ones

Right now if we bind-mount / of some FS into container and want this
to be 'external' mount, criu would refuse to dump this mountpoint.

Tune the check in validate_mounts() to always accept any external mps
regardless of whether they are fsroot or not.

https://github.com/xemul/criu/issues/93



Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Tested-by: Dengguangxing <dengguangxing@huawei.com>