gru: add user request to explicitly unload a gru context

Add user function to explicitly unload GRU kernel contexts from the GRU.
Only contexts that are not in-use will be unloaded.

This function is primarily for testing.  It is not expected that this will
be used in normal production systems.

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>