_create_and_load_v4: propagate ioctl errors back to caller

_create_and_load_v4: propagate ioctl errors back to caller

When setting up dm-verity devices with signed root hashes it is very
useful to have a recognizable error code when a key is not present in
the kernel keyring. Turns out the kernel actually returns ENOKEY in that
case, but this gets lost in libdevmapper.

This fixes this: in _create_and_load_v4() it copies the error code from
the ioctl from the sub-tasks back to the main task field on failure.

This is not enough to make libcryptsetup actually propagate the ENOKEY
correctly, that also needs a patch to libcryptsetup, but this is part of
the puzzle.