iommu/tegra-smmu: Use kcalloc() to allocate counter array

Use kcalloc() to allocate the use-counter array for the page directory
entries/page tables.  Using kcalloc() allows us to be provided with
zero-initialised memory from the allocators, rather than initialising
it ourselves.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Thierry Reding <treding@nvidia.com>