cryptsetup now requires dm-crypt v1.18.1 or higher
to use kernel keyring service for passing VKs.
Also, relevant API functions fail if CRYPT_ACTIVATE_KEYRING_KEY
is set, but library is not allowed to use kernel keyring for
VK.
This patch duplicates part of the code because following
switch to non-recursive automake is not easily fixable without this change.
(Automake cannot use top_srcdir anymore in this context.)