s390/dasd: Avoid field over-reading memcpy()

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field array bounds checking for memcpy(), memmove(), and memset(),
avoid intentionally reading across neighboring array fields.

Add a wrapping structure to serve as the memcpy() source, so the compiler
can do appropriate bounds checking, avoiding this future warning:

In function '__fortify_memcpy',
    inlined from 'create_uid' at drivers/s390/block/dasd_eckd.c:749:2:
./include/linux/fortify-string.h:246:4: error: call to '__read_overflow2_field' declared with attribute error: detected read beyond size of field (2nd parameter)

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://lore.kernel.org/r/20210701142221.3408680-3-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>