mount: prevent loop mounting the same file twice

mount: prevent loop mounting the same file twice

The mount syscall prevents mounting the same device twice
to the same mountpoint. When loop mounting a file, for each
file a new loop device gets allocated, which prevents the detection
of loop mounting the same file to the same mountpoint twice.
The patch adds a check to prevent double mounts, if the same loopfile
is going to be mounted with the same offset to the same mountpoint.

Co-Author: Matthias Koenig <mkoenig@suse.de>
Signed-off-by: Matthias Koenig <mkoenig@suse.de>
Signed-off-by: Karel Zak <kzak@redhat.com>