powerpc/spufs: reference context while dropping state mutex in scheduler

Based on an original patch from Christoph Hellwig <hch@lst.de>.

Currently, there is a possible reference-after-free in the spusched
code - contexts may be freed after we have released their state_mutex
in spusched_tick and find_victim.

This change takes a reference to the context before releasing the
mutex, so that the context doesn't get destroyed.

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