[POWERPC] spufs: Fix error handling in spufs_fill_dir()

The error path in spufs_fill_dir() is broken. If d_alloc_name() or
spufs_new_file() fails, spufs_prune_dir() is getting called. At this time
dir->inode is not set and a NULL pointer is dereferenced by mutex_lock().
This bugfix replaces spufs_prune_dir() with a shorter version that does
not touch dir->inode but simply removes all children.

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