iommu/arm-smmu-v3: Avoid open coded arithmetic in memory allocation

kmalloc_array()/kcalloc() should be used to avoid potential overflow when
a multiplication is needed to compute the size of the requested memory.

So turn a devm_kzalloc()+explicit size computation into an equivalent
devm_kcalloc().

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/3f7b9b202c6b6f5edc234ab7af5f208fbf8bc944.1644274051.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Will Deacon <will@kernel.org>