dm: dm-zoned: Adjust dmz_load_mapping() allocation type

dm: dm-zoned: Adjust dmz_load_mapping() allocation type

In preparation for making the kmalloc family of allocators type aware,
we need to make sure that the returned type from the allocation matches
the type of the variable being assigned. (Before, the allocator would
always return "void *", which can be implicitly cast to any pointer type.)

The assigned type is "struct dmz_mblock **" but the returned type will
be "struct dmz_mblk **". These are the same allocation size (pointer
size), but the types do not match. Adjust the allocation type to match
the assignment.

Link: https://patch.msgid.link/20250426061707.work.587-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>