drm/gpusvm: fix hmm_pfn_to_map_order() usage

In the Linux kernel, the following vulnerability has been resolved: drm/gpusvm: fix hmm_pfn_to_map_order() usage Handle the case where the hmm range partially covers a huge page (like 2M), otherwise we can potentially end up doing something nasty like mapping memory which is outside the range, and maybe not even mapped by the mm. Fix is based on the xe userptr code, which in a future patch will directly use gpusvm, so needs alignment here. v2: - Add kernel-doc (Matt B) - s/fls/ilog2/ (Thomas)