iommu/amd: Remove latent out-of-bounds access in IOMMU debugfs

In the Linux kernel, the following vulnerability has been resolved:

iommu/amd: Remove latent out-of-bounds access in IOMMU debugfs

In iommu_mmio_write() and iommu_capability_write(), the variables
dbg_mmio_offset and dbg_cap_offset are declared as int. However, they
are populated using kstrtou32_from_user(). If a user provides a
sufficiently large value, it can become a negative integer.

Prior to this patch, the AMD IOMMU debugfs implementation was already
protected by different mechanisms.

1. #define OFS_IN_SZ 8 ensures the user string <= 8 bytes, so
e.g. 0xffffffff isn't a valid input.

if (cnt > OFS_IN_SZ)
return -EINVAL;

2. Implicit type promotion in iommu_mmio_write(), dbg_mmio_offset is int
and iommu->mmio_phys_end is u64

if (dbg_mmio_offset > iommu->mmio_phys_end - sizeof(u64))
return -EINVAL;

3. The show handlers would currently catch the negative number and
refuse to perform the read.

Replace kstrtou32_from_user() with kstrtos32_from_user() to parse the
input, and check for negative values to explicitly prevent out-of-bounds
memory accesses directly in iommu_mmio_write() and
iommu_capability_write().