drm - Refactor task_struct and implement mm_struct

drm - Refactor task_struct and implement mm_struct

* Change td->td_linux_task from an embedded structure to a pointer.

* Add p->p_linux_mm to support tracking mm_struct's.

* Change the 'current' macro to test td->td_linux_task and call
  a support function, linux_task_alloc(), if it is NULL.

* Implement callbacks from the main kernel for thread exit and
  process exit to support functions that drop the td_linux_task and
  p_linux_mm pointers.

  Initialize and clear these callbacks in the module load/unload
  in drm_drv.c

* Implement required support functions in linux_sched.c