dm-verity: introduce the options restart_on_error and panic_on_error

This patch introduces the options restart_on_error and panic_on_error on
dm-verity.

Previously, restarting on error was handled by the patch
e6a3531d, but Google engineers wanted to
have a special option for it.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Suggested-by: Sami Tolvanen <samitolvanen@google.com>
Suggested-by: Will Drewry <wad@chromium.org>