kernel - Don't block in tstop() with locks held

kernel - Don't block in tstop() with locks held

* There are several places where the kernel improperly blocks on a
  STOP signal while locks might be held.  This is a particular problem
  when PCATCH is specified e.g. in the middle of the NFS code.  It is
  meant to catch INTR but it also improperly allowed STOP to function
  and left the vnode lock held.

  Several other places in the kernel also use PCATCH and don't expect
  the kernel to actually block indefinitely on a STOP.

* Don't block in STOP in these situations.  Simply mark the thread as
  stopped and wait until it tries to return to userland before actually
  stopping.

  Any kernel subsystems which desire to act on the STOP in-line instead
  of upon return to userland can do so manually, as long as they release
  all locks for the duration.