partitions/ibm: Introduce defines for magic string length values

partitions/ibm: Introduce defines for magic string length values

The length values for volume label type and volume label id are
hard-coded in several places. Provide defines for those values and
replace all occurrences accordingly.

Note that the length is defined and used, and not the size since the
volume label type string and volume label id string are not
nul-terminated.

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/20230915131001.697070-4-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>