[POWERPC] spufs: Fix context destroy vs /spu readdir race

We can currently cause an oops by repeatedly creating and destroying
contexts, while doing getdents() calls on the "/spu" directory.

This is due to the context's top-level dentry remaining hashed while
the context is being destroyed.

Fix this by unhashing the context's dentry with the
dentry->d_inode->i_mutex held. This way, we'll hit the check for
d_unhashed in dentry_readdir, and won't be included in the
list of subdirs for /spu.

test: spufs-testsuite:tests/01-spu_create/07-destroy-vs-readdir-race
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/platforms/cell/spufs/inode.c

@@ -171,6 +171,7 @@ static int spufs_rmdir(struct inode *parent, struct dentry *dir)
 {
 	/* remove all entries */
 	spufs_prune_dir(dir);
+	d_drop(dir);
 
 	return simple_rmdir(parent, dir);
 }