fuse: fix nlink after unlink

Anand Avati reports that the following sequence of system calls fail on a fuse
filesystem:


 	create("filename") => 0
 	link("filename", "linkname") => 0
 	unlink("filename") => 0
 	link("linkname", "filename") => -ENOENT ### BUG ###

vfs_link() fails with ENOENT if i_nlink is zero, this is done to prevent
resurrecting already deleted files.

Fuse clears i_nlink on unlink even if there are other links pointing to the
file.
Reported-by: Anand Avati <avati@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/dir.c

@@ -644,13 +644,12 @@ static int fuse_unlink(struct inode *dir, struct dentry *entry)
 	fuse_put_request(fc, req);
 	if (!err) {
 		struct inode *inode = entry->d_inode;
+		struct fuse_inode *fi = get_fuse_inode(inode);
 
-		/*
-		 * Set nlink to zero so the inode can be cleared, if the inode
-		 * does have more links this will be discovered at the next
-		 * lookup/getattr.
-		 */
-		clear_nlink(inode);
+		spin_lock(&fc->lock);
+		fi->attr_version = ++fc->attr_version;
+		drop_nlink(inode);
+		spin_unlock(&fc->lock);
 		fuse_invalidate_attr(inode);
 		fuse_invalidate_attr(dir);
 		fuse_invalidate_entry_cache(entry);
@@ -762,8 +761,17 @@ static int fuse_link(struct dentry *entry, struct inode *newdir,
 	   will reflect changes in the backing inode (link count,
 	   etc.)
 	*/
-	if (!err || err == -EINTR)
+	if (!err) {
+		struct fuse_inode *fi = get_fuse_inode(inode);
+
+		spin_lock(&fc->lock);
+		fi->attr_version = ++fc->attr_version;
+		inc_nlink(inode);
+		spin_unlock(&fc->lock);
 		fuse_invalidate_attr(inode);
+	} else if (err == -EINTR) {
+		fuse_invalidate_attr(inode);
+	}
 	return err;
 }