KVM: MMU: Clean up gpte reading with copy_from_user()

When we optimized walk_addr_generic() by not using the generic guest
memory reader, we replaced copy_from_user() with get_user():

  commit e30d2a170506830d5eef5e9d7990c5aedf1b0a51
  KVM: MMU: Optimize guest page table walk

  commit 15e2ac9a43d4d7d08088e404fddf2533a8e7d52e
  KVM: MMU: Fix 64-bit paging breakage on x86_32

But as Andi pointed out later, copy_from_user() does the same as
get_user() as long as we give a constant size to it.

So we use copy_from_user() to clean up the code.

The only, noticeable, regression introduced by this is 64-bit gpte
reading on x86_32 hosts needed for PAE guests.

But this can be mitigated by implementing 8-byte get_user() for x86_32,
if needed.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>