accel/amdxdna: Fix locally exploitable BUG_ON in amdxdna_insert_pages()

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

accel/amdxdna: Fix locally exploitable BUG_ON in amdxdna_insert_pages()

In amdxdna_insert_pages(), vm_flags_mod() sets VM_MIXEDMAP and clears
VM_PFNMAP. If an unprivileged userspace process mmaps a non-imported GEM
object and then calls madvise(MADV_DONTNEED), the PTEs will be
successfully cleared because VM_MIXEDMAP allows this (unlike VM_PFNMAP).

When userspace subsequently accesses the memory, drm_gem_shmem_fault()
handles the page fault and attempts to map the backing shmem page via
vmf_insert_pfn() which calls vmf_insert_pfn_prot(). Because the backing
shmem page is normal system memory (pfn_valid(pfn) is true) and the VMA
now has VM_MIXEDMAP set, won't this predictably trigger the explicit
assertion BUG_ON((vma->vm_flags & VM_MIXEDMAP) && pfn_valid(pfn))

Fix by removing the vm_flags_mod() call and replacing the vm_insert_pages()
pre-population with the handle_mm_fault() loop that was already used for
the import (dma-buf) path.