UPSTREAM: dm verity: separate function for parsing opt args

Move optional argument parsing into a separate function to make it
easier to add more of them without making verity_ctr even longer.

Change-Id: I9cd9df41c3326824f8cca5764075501987e78a52
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit 753c1fd02807cb43a1c5d01d75d454054d46bdad)