libbb/loop: don't try to re-use existing loop device

libbb/loop: don't try to re-use existing loop device

loopinfo.lo_file_name is not enough to uniquely identify a file on a system with
multiple mount namespaces.  We could conceivably change this to dedup on
(lo_rdevice, lo_inode), but, as the comment above the deleted code notes, this
whole approach of reusing devices is racy anyway, so it seems better to stop
doing it entirely.

Signed-off-by: Kevin Wallace <k@igneous.io>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>