gru: add comments raised in previous code reviews

Add comments from previous code reviews.  The comments help explain some
of the more esoteric aspects of the driver.

Move a free() to the other side of an unlock.

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>