common.sh: Add retry logic to loopback_partscan

We have continually had issues with mounting and
unmounting loopback paritions via partx. This commit
adds a mechanism to retry as well as fully detach
and reattach using losetup.

BUG=chromium:1011891
TEST=tryjob

Change-Id: I7153aaec512c70493b4e42adce4472de479ad322
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1885394
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: George Engelbrecht <engeg@google.com>
Tested-by: George Engelbrecht <engeg@google.com>
Auto-Submit: George Engelbrecht <engeg@google.com>
(cherry picked from commit fcd236f4029d0e65d0870da789096b5d97cffa02)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1899052
1 file changed