This function ensures the argument is the mount point
(i.e. if it's not, it bind mounts it to itself).

Signed-off-by: Kir Kolyshkin <[email protected]>

For some reason, shared mount propagation between the host
and a container does not work for btrfs, unless container
root directory (i.e. graphdriver home) is a bind mount.

The above issue was reproduced on SLES 12sp3 + btrfs using
the following script:

	#!/bin/bash
	set -eux -o pipefail

	# DIR should not be under a subvolume
	DIR=${DIR:-/lib}
	MNT=$DIR/my-mnt
	FILE=$MNT/file

	ID=$(docker run -d --privileged -v $DIR:$DIR:rshared ubuntu sleep 24h)
	docker exec $ID mkdir -p $MNT
	docker exec $ID mount -t tmpfs tmpfs $MNT
	docker exec $ID touch $FILE
	ls -l $FILE
	umount $MNT
	docker rm -f $ID

which fails this way:

	+ ls -l /lib/my-mnt/file
	ls: cannot access '/lib/my-mnt/file': No such file or directory

meaning the mount performed inside a priviledged container is not
propagated back to the host (even if all the mounts have "shared"
propagation mode).

The remedy to the above is to make graphdriver home a bind mount.

Signed-off-by: Kir Kolyshkin <[email protected]>