libmount: reuse existing loop device

libmount: reuse existing loop device

According to the Al Viro[1], kernel has no way to detect that a single file is
used by multiple loop devices, and multiple mounts of the same file using
different loop devices will result in a data corruption. Exactly this now
happens, if multiple btrfs sub-volumes in one file are mounted with "-oloop".

Make use of multiple -oloop mounting the same file safe: Do a loop devices
lookup, and if a loop device is already initialized, use it.

Hopefully it is possible, as "losetup -d" will return OK, even if the device
itself is in use, and is not released.

Problems:

There is a risk of race condition between the lookup and real mount.

Once loop device is initialized read-only, kernel offers no way to turn it to
read-write. It has to fail.

References:
https://lkml.org/lkml/2016/2/26/897

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>