gru: allow users to specify gru chiplet 3

This patch builds on the infrastructure introduced in the patches that
allow user specification of GRU blades & chiplets for context allocation.

This patch simplifies the algorithms for migrating GRU contexts between
blades.

No new functionality is introduced.

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>