The 4 level pagetable code changed the exit_mmap code to rely on TASK_SIZE.
 On some architectures (eg ppc64 and ia64), this is a per task property and
bad things can happen in certain circumstances when using it.

It is possible for one task to end up "owning" an mm from another - we have
seen this with the procfs code when process 1 accesses /proc/pid/cmdline of
process 2 while it is exiting.  Process 2 exits but does not tear its mm
down.  Later on process 1 finishes with the proc file and the mm gets torn
down at this point.

Now if process 1 was 32bit and process 2 was 64bit then we end up using a
bad value for TASK_SIZE in exit_mmap.  We only tear down part of the
address space and leave half initialised pagetables and entries in the MMU
etc.

MM_VM_SIZE() was created for this purpose (and is used in the next line for
tlb_finish_mmu), so use it.  I moved the PGD round up of TASK_SIZE into the
default MM_VM_SIZE.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>