fs/proc/task_mmu.c: don't use task->mm in m_start() and show_*map()

fs/proc/task_mmu.c: don't use task->mm in m_start() and show_*map()

get_gate_vma(priv->task->mm) looks ugly and wrong, task->mm can be NULL or
it can changed by exec right after mm_access().

And in theory this race is not harmless, the task can exec and then later
exit and free the new mm_struct.  In this case get_task_mm(oldmm) can't
help, get_gate_vma(task->mm) can read the freed/unmapped memory.

I think that priv->task should simply die and hold_task_mempolicy() logic
can be simplified.  tail_vma logic asks for cleanups too.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>