vfs: restructure d_genocide()

It shouldn't matter when we decrement the refcount during the walk as long
as we do it exactly once.

Restructure d_genocide() to do the killing on entering the dentry instead
of when leaving it.  This helps creating a common helper for tree walking.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -2952,25 +2952,21 @@ void d_genocide(struct dentry *root)
 			spin_unlock(&dentry->d_lock);
 			continue;
 		}
+		if (!(dentry->d_flags & DCACHE_GENOCIDE)) {
+			dentry->d_flags |= DCACHE_GENOCIDE;
+			dentry->d_lockref.count--;
+		}
 		if (!list_empty(&dentry->d_subdirs)) {
 			spin_unlock(&this_parent->d_lock);
 			spin_release(&dentry->d_lock.dep_map, 1, _RET_IP_);
 			this_parent = dentry;
 			spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);
 			goto repeat;
 		}
-		if (!(dentry->d_flags & DCACHE_GENOCIDE)) {
-			dentry->d_flags |= DCACHE_GENOCIDE;
-			dentry->d_lockref.count--;
-		}
 		spin_unlock(&dentry->d_lock);
 	}
 	if (this_parent != root) {
 		struct dentry *child = this_parent;
-		if (!(this_parent->d_flags & DCACHE_GENOCIDE)) {
-			this_parent->d_flags |= DCACHE_GENOCIDE;
-			this_parent->d_lockref.count--;
-		}
 		this_parent = try_to_ascend(this_parent, locked, seq);
 		if (!this_parent)
 			goto rename_retry;