iommu/omap: Fix buffer overflow in debugfs

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

iommu/omap: Fix buffer overflow in debugfs

There are two issues here:

1) The "len" variable needs to be checked before the very first write.

Otherwise if omap2_iommu_dump_ctx() with "bytes" less than 32 it is a

buffer overflow.

2) The snprintf() function returns the number of bytes that *would* have

been copied if there were enough space. But we want to know the

number of bytes which were *actually* copied so use scnprintf()

instead.