[POWERPC] spufs: Free mm if spufs_fill_dir() failed

In case spufs_fill_dir() fails only put_spu_context()
gets called for cleanup and the acquired mm_struct never gets freed.

Signed-off-by: Sebastian Siewior <bigeasy@linux.vnet.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>