gru: update gru kernel self tests

Change the kernel self tests that can be optionally executed on GRU
initialization.  This is primarily for testing.

Eliminate the BUG statements on failure and return bad status.  Add ioctl
interface to execute the tests on demand.

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>