__unhash_process acquires the dcache_lock while holding the
tasklist_lock for writing. This can deadlock. Additionally,
fs/proc/base.c incorrectly assumed that p->pid would be set to 0 during
release_task.

The patch fixes that by adding a new spinlock to the task structure and
fixing all references to (!p->pid).

The alternative to the new spinlock would be to hold dcache_lock around
__unhash_process.

- fs/proc/base.c assumed that p->pid is reset to 0 during exit.  This is
  not the case anymore.  I now look at the count of the pid structure for
  PIDTYPE_PID.

- de_thread now tested - as broken as it was before: open handles to
  /proc/<pid> are either stale or invalid after an exec of a nptl process,
  if the exec was call from a secondary thread.

- a few lock_kernels removed - that part of /proc doesn't need it.

- additional instances of 'if(current->pid)' replaced with pid_alive.