[pve-devel] avoid mointing same filesystem multiple times

[Dietmar Maurer]

Seems there is no protection against this dangerous situation:

# qemu-img create -f raw test.raw 4G
# mkfs.ext4 test.raw
# mkdir mp1
# mkdir mp2
# mount -o loop test.raw mp1
# mount -o loop test.raw mp2

So how can we make sure that this never happens? We use loop
mounts in our pct code.