cryptsetup: try to load bitlocker superblock only if requested

cryptsetup: try to load bitlocker superblock only if requested

let's do automatic discovery only for our native LUKS/LUKS2 headers,
since they are Linux stuff, and let's require that BitLocker to be
requested explicitly.

This makes sure cryptsetup without either "luks" nor "bitlk" in the
option string will work. Right now it would fail because we'd load the
superblock once with luks and once with bitlk and one of them would
necessarily fail.

Follow-up for #15979