[POWERPC] spufs: fix context destruction during psmap fault

We have a small window where a spu context may be destroyed while
we're servicing a page fault (from another thread) to the context's
problem state mapping.

After we up_read() the mmap_sem, it's possible that the context is
destroyed by its owning thread, and so the later references to ctx
are invalid. This can maifest as a deadlock on the (now free()-ed)
context state mutex.

This change adds a reference to the context before we release the
mmap_sem, so that the context cannot be destroyed.

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