libmount: one iteration to detect overlap and reuse loopdev

libmount: one iteration to detect overlap and reuse loopdev

The current code scans loopdevs to detect already used loop device and
another scan to detect overlap.

Let's use one scan only, for this purpose loopcxt_find_overlap() has
been modified to return info (rc==2) about full size and offset match.

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