powerpc/spufs: Fix multiple get_spu_context()

powerpc/spufs: Fix multiple get_spu_context()

Commit 76c94211f1ac076343be4da596b8992ecad9f6d4 introduced a reference
count on SPU contexts during find_victim, but this may cause a leak in
the reference count if we later find a better contender for a context to
unschedule.

Change the reference to after we've found our victim context, so we
don't do the extra get_spu_context().

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>