libmount: append loop option when automatically create loopdev

libmount: append loop option when automatically create loopdev

The loop option is optional, mount(8) is able to detect that the
source path is regular file (image) with known filesystem -- then a
loop device is automatically created. In this case we have to store
"loop" option to mtab on systems without autoclear loopdev flag.

Signed-off-by: Karel Zak <kzak@redhat.com>