block: constify partition prober array

The array is never modified so it can be const.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202304191640.SkNk7kVN-lkp@intel.com/


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20230419-const-partition-v3-1-4e14e48be367@weissschuh.net


Signed-off-by: Jens Axboe <axboe@kernel.dk>