lguest: Revert 31859cb106109f80eabe1937004d3d804e373e70, fix real problem.

lguest: Revert 31859cb106109f80eabe1937004d3d804e373e70, fix real problem.

Ahmed managed to crash the Host in release_pgd(), which cannot be a Guest
bug, and indeed it wasn't.

The bug was that handing a 0 as the address of the toplevel page table
being manipulated can cause the lookup code in find_pgdir() to return
an uninitialized cache entry (we shadow up to 4 top level page tables
for each Guest).

Commit 92d2679d388a5a00b884830bb7901ef6b7528d8e introduced this
behaviour in the Guest, uncovering the bug.

The patch which he submitted (which removed the /4 from the index
calculation) simply ensured that these high-indexed entries hit the
early exit path of guest_set_pmd().  But you get lots of segfaults in
guest userspace as the PMDs aren't being updated.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>