fuse: verify nlink

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>

fs/fuse/dir.c

@@ -870,7 +870,8 @@ static int fuse_link(struct dentry *entry, struct inode *newdir,
 
 		spin_lock(&fi->lock);
 		fi->attr_version = atomic64_inc_return(&fc->attr_version);
-		inc_nlink(inode);
+		if (likely(inode->i_nlink < UINT_MAX))
+			inc_nlink(inode);
 		spin_unlock(&fi->lock);
 		fuse_invalidate_attr(inode);
 		fuse_update_ctime(inode);