1. send_sigio() sends to a specific thread, _not_ a process.
   (It can also send to a process group, but that's not relevant here).
   This is useful, and should stay as it is.

   Therefore it makes _no sense_ to call f_setown() with current->tgid.
   Presently the kernel is inconsistent about it, with some places using
   current->pid and some others using current->tgid.

   This patch changes f_setown() calls to use current->pid.

2. In some places, f_setown() is called not at the user's direct request,
   but as a side effect of another function.  Specifically: dnotify and
   file leases.

   It is good to allow a program the flexibility to specify a different
   pid than the default, using F_SETOWN.  Presently they can do this after
   the dnotify or lease call, but there is a small time window when it
   will be temporarily set to current->tgid (which as pointed out above,
   is not always right).

   The window is avoidable if the program can use F_SETOWN prior to the
   dnotify or lease call.  This is exactly what the "force" argument to
   f_setown() is for, and this patch changes it to zero in those callers.

   This change is not likely to affect any existing programs.