s390/dasd: Simplify uid string generation

s390/dasd: Simplify uid string generation

There are two variants of the device uid string. One containing the
virtual device unit information table (vduit) identifying the device as
a virtual device located on a real device in a z/VM environment. The
other variant does not contain those additional information.

Simplify the string generation with a shorter check of an existing vduit
embedded in the snprintf() calls.

Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://lore.kernel.org/r/20240208164248.540985-2-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>