mount_gpt_image.sh: try harder to unmount in some cases

If we're unmounting a disk image and we don't have easy access to the
GPT script/details that created the mounts, just use a plain recursive
unmount call to clean things up.

BUG=None
TEST=precq passes
TEST=`./mount_gpt_image.sh -i $PWD/chromiumos_base_image.bin` works
TEST=`./mount_gpt_image.sh -u -i $PWD/chromiumos_base_image.bin` works

Change-Id: If1c02802a7f5a612610604437905403fb8502f90
Reviewed-on: https://chromium-review.googlesource.com/1034230
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@google.com>
1 file changed