drm: buddy_allocator: Fix buddy allocator init on 32-bit systems

The drm buddy allocator tests were broken on 32-bit systems, as
rounddown_pow_of_two() takes a long, and the buddy allocator handles
64-bit sizes even on 32-bit systems.

This can be reproduced with the drm_buddy_allocator KUnit tests on i386:
./tools/testing/kunit/kunit.py run --arch i386 \
--kunitconfig ./drivers/gpu/drm/tests drm_buddy

(It results in kernel BUG_ON() when too many blocks are created, due to
the block size being too small.)

This was independently uncovered (and fixed) by Luís Mendes, whose patch
added a new u64 variant of rounddown_pow_of_two(). This version instead
recalculates the size based on the order.

Reported-by: Luís Mendes <[e-mail]>
Link: https://lore.kernel.org/lkml/CAEzXK1oghXAB_KpKpm=[e-mail]/T/
Signed-off-by: David Gow <[e-mail]>
Acked-by: Christian König <[e-mail]>
Reviewed-by: Arunpravin Paneer Selvam <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Christian König <[e-mail]>

drivers/gpu/drm/drm_buddy.c

···
 		unsigned int order;
 		u64 root_size;
 
-		root_size = rounddown_pow_of_two(size);
-		order = ilog2(root_size) - ilog2(chunk_size);
+		order = ilog2(size) - ilog2(chunk_size);
+		root_size = chunk_size << order;
 
 		root = drm_block_alloc(mm, NULL, order, offset);
 		if (!root)