mnt: cut shared root path from bind path

If a mount like:

96 95 0:21 /cgmanager /sys/fs/cgroup/cgmanager rw master:9 - tmpfs tmpfs rw,mode=755

is present in the container and the host has a similar bind mount, e.g.

46 27 0:21 /cgmanager /sys/fs/cgroup/cgmanager rw shared:9 - tmpfs tmpfs rw,mode=755

then the best match mount's root path /and/ the target mountpoint have part of
the path in them; we should cut the shared piece of the path and just
concatenate the non-duplicate pieces.

Reported-by: Andrew Vagin <avagin@odin.com>
CC: Andrew Vagin <avagin@odin.com>
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>