fs: dcache scale dentry refcount

Make d_count non-atomic and protect it with d_lock. This allows us to ensure a
0 refcount dentry remains 0 without dcache_lock. It is also fairly natural when
we start protecting many other dentry members with d_lock.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
diff --git a/fs/nilfs2/super.c b/fs/nilfs2/super.c
index f804d41..d36fc7e 100644
--- a/fs/nilfs2/super.c
+++ b/fs/nilfs2/super.c
@@ -838,7 +838,7 @@
 
 static int nilfs_tree_was_touched(struct dentry *root_dentry)
 {
-	return atomic_read(&root_dentry->d_count) > 1;
+	return root_dentry->d_count > 1;
 }
 
 /**