gru: change resource assignment for kernel threads

Change the way GRU resources are assigned for kernel threads.  GRU
contexts for kernel threads are now allocated on demand and can be stolen
by user processes when idle.  This allows MPI jobs to use ALL of the GRU
resources when the kernel is not using them.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>