debugfs: Fix statfs() regression in 3.2.69

Commit 915f4f86 ("debugfs: leave freeing a symlink body until inode
eviction", commit 0db59e59 upstream) changed debugfs to define its
own super_operations and implement the evict_inode operation.

Luis Henriques pointed out that it needs to define the statfs
operation, as in simple_super_operations which it was using before.
Reported-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/debugfs/inode.c

@@ -135,6 +135,7 @@ static void debugfs_evict_inode(struct inode *inode)
 
 static const struct super_operations debugfs_super_operations = {
 	.evict_inode	= debugfs_evict_inode,
+	.statfs		= simple_statfs,
 };
 
 static int debug_fill_super(struct super_block *sb, void *data, int silent)