iommu/amd: Fix potential out-of-bounds read in iommu_mmio_show

In iommu_mmio_write(), it validates the user-provided offset with the
check: `iommu->dbg_mmio_offset > iommu->mmio_phys_end - 4`.
This assumes a 4-byte access. However, the corresponding
show handler, iommu_mmio_show(), uses readq() to perform an 8-byte
(64-bit) read.

If a user provides an offset equal to `mmio_phys_end - 4`, the check
passes, and will lead to a 4-byte out-of-bounds read.

Fix this by adjusting the boundary check to use sizeof(u64), which
corresponds to the size of the readq() operation.

Fixes: 7a4ee419 ("iommu/amd: Add debugfs support to dump IOMMU MMIO registers")
Signed-off-by: Songtang Liu <liusongtang@bytedance.com>
Reviewed-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
Tested-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>