iommu: dma-mapping-fast: Add a check for count in fast_smmu_alloc

In fast_smmu_alloc size_t variable size is type casted to int
variable count, this variable count can get truncated and can
result in memory corruption during unmap, make count as size_t
and also add a check for count as sg_alloc_table_from_pages
accepts unsigned int value for count.

Change-Id: I4780a554c5c062fd9dd229e5cc0ac804b1ba31d8
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>