gru: support cch_allocate for kernel threads

Change the interface to cch_allocate so that it can be used to allocate
GRU contexts for kernel threads.  Kernel threads use the GRU in unmapped
mode and do not require ASIDs for the GRU TLB.

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>