exit_mmap() currently assumes that the exitting task used virtual address
span TASK_SIZE.

But on some platforms, TASK_SIZE is variable, based on current->mm.

But exit_mmap() can be called from (say) procfs's call to mmput.  In which
case current->mm has nothing to do with the mm which is being put in
mmput().

So rather than assuming that the mm which is being put is current->mm, we
need to calculate the virtual span of the mm.  Add a new per-arch macro
MM_VM_SIZE() for that.

Some platforms can currently go BUG over this (where?).  sparc64 is safe
because our TASK_SIZE is constant.

Platforms such as ia64 should stick the VM extent inside of mm_struct, I'd
suggest adding it to mm_context_t.

1) TASK_SIZE means what is valid for mmap()'s in the processes
   address space

2) MM_VM_SIZE means where things might be mapped for a MM, including
   private implementation-specific areas created by the kernel
   which the user cannot access