fuse: verify nlink

BugLink: https://bugs.launchpad.net/bugs/1858489

commit c634da71 upstream.

When adding a new hard link, make sure that i_nlink doesn't overflow.

Fixes: ac45d613 ("fuse: fix nlink after unlink")
Cc: <stable@vger.kernel.org> # v3.4
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/fuse/dir.c

@@ -814,7 +814,8 @@ static int fuse_link(struct dentry *entry, struct inode *newdir,
 
 		spin_lock(&fc->lock);
 		fi->attr_version = ++fc->attr_version;
-		inc_nlink(inode);
+		if (likely(inode->i_nlink < UINT_MAX))
+			inc_nlink(inode);
 		spin_unlock(&fc->lock);
 		fuse_invalidate_attr(inode);
 		fuse_update_ctime(inode);