binder: rename alloc->vma_vm_mm to alloc->mm

binder: rename alloc->vma_vm_mm to alloc->mm

Rename ->vma_vm_mm to ->mm to reflect the fact that we no longer cache
this reference from vma->vm_mm but from current->mm instead.

No functional changes in this patch.

Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Acked-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20220906135948.3048225-2-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>