[POWERPC] spufs: don't acquire state_mutex interruptible while performing callback

There's currently no way to tell if spu_process_callback has
returned with the state mutex held, as -EINTR may be returned
by either the syscall or the spu_acquire fail case.

Instead, just do a non-interruptible mutex_lock here.

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