[POWERPC] spufs: Hook up spufs_release_mem

Currently spufs_mem_release and the mem file doesn't have any release
method hooked up, leading to leaks everytime is used.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>