dm crypt: update wiki page URL

Cryptsetup moved to gitlab.  This is a leftover from commit e44f23b3


(dm crypt: update URLs to new cryptsetup project page, 2015-04-05).

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>