[POWERPC] spufs: clear mapping pointers after last close

Make sure the pointers to various mappings are cleared once the last
user stopped using them.  This avoids accessing freed memory when
tearing down the gang directory aswell as optimizing away
pte invalidations if no one uses these.

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