[POWERPC] spufs: turn run_sema into run_mutex

There is no reason for run_sema to be a struct semaphore.  Changing
it to a mutex and rename it accordingly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>