[POWERPC] spufs: fix memory leak on spufs reloading

When SPU isolation mode enabled, isolated_loader would be
allocated by spufs_init_isolated_loader() on module_init().
But anyone do not free it.

This patch introduces spufs_exit_isolated_loader() which is
the opposite of spufs_init_isolated_loader() and called on
module_exit().

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Akinobu Mita <mita@fixstars.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>