Added a new "vm -M <mm_struct>" option.  When a task is exiting,
the mm_struct address pointer in its task_struct is NULL'd out, and
as a result, the "vm" command looks like this:

  crash> vm
  PID: 4563   TASK: ffff88049863f500  CPU: 8   COMMAND: "postgres"
         MM               PGD          RSS    TOTAL_VM
         0                 0            0k       0k

However, the mm_struct address can be retrieved from the task's
kernel stack and entered manually with this option, which allows the
"vm" command to attempt to dump the virtual memory data of the task.
It may, or may not, work, depending upon how far the virtual memory
deconstruction has proceeded.  This option only verifies that the
address entered is from the "mm_struct" slab cache, and that
its mm_struct.mm_count is non-zero.
(qiaonuohan@cn.fujitsu.com, anderson@redhat.com)