fuse: fix pipe_buf_operations

Having this struct in module memory could Oops when if the module is
unloaded while the buffer still persists in a pipe.

Since sock_pipe_buf_ops is essentially the same as fuse_dev_pipe_buf_steal
merge them into nosteal_pipe_buf_ops (this is the same as
default_pipe_buf_ops except stealing the page from the buffer is not
allowed).

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

fs/splice.c

@@ -555,6 +555,24 @@ static const struct pipe_buf_operations default_pipe_buf_ops = {
 	.get = generic_pipe_buf_get,
 };
 
+static int generic_pipe_buf_nosteal(struct pipe_inode_info *pipe,
+				    struct pipe_buffer *buf)
+{
+	return 1;
+}
+
+/* Pipe buffer operations for a socket and similar. */
+const struct pipe_buf_operations nosteal_pipe_buf_ops = {
+	.can_merge = 0,
+	.map = generic_pipe_buf_map,
+	.unmap = generic_pipe_buf_unmap,
+	.confirm = generic_pipe_buf_confirm,
+	.release = generic_pipe_buf_release,
+	.steal = generic_pipe_buf_nosteal,
+	.get = generic_pipe_buf_get,
+};
+EXPORT_SYMBOL(nosteal_pipe_buf_ops);
+
 static ssize_t kernel_readv(struct file *file, const struct iovec *vec,
 			    unsigned long vlen, loff_t offset)
 {