NOMMU: Don't need to clear vm_mm when deleting a VMA

Don't clear vm_mm in a deleted VMA as it's unnecessary and might
conceivably break the filesystem or driver VMA close routine.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/nommu.c

@@ -779,8 +779,6 @@ static void delete_vma_from_mm(struct vm_area_struct *vma)
 
 	if (vma->vm_next)
 		vma->vm_next->vm_prev = vma->vm_prev;
-
-	vma->vm_mm = NULL;
 }
 
 /*