dmapool: speedup DMAPOOL_DEBUG with init_on_alloc

Avoid double-memset of the same allocated memory in dma_pool_alloc() when
both DMAPOOL_DEBUG is enabled and init_on_alloc=1.

Link: https://lkml.kernel.org/r/20230126215125.4069751-6-kbusch@meta.com
Fixes: 2d55c16c

 ("dmapool: create/destroy cleanup")
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>