UPSTREAM: dm verity: separate function for parsing opt args

(cherry-picked from 753c1fd02807cb43a1c5d01d75d454054d46bdad)

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

Bug: 21893453
Change-Id: I61ab0a659fdbd67acb8f04bc43f14b9613f7a0a8
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>