Teach sendfile() to honour non-seekable source files.

Remove the test from the socket code, now that it's no
longer necessary.

fs/read_write.c

@@ -567,8 +567,12 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
 		goto fput_in;
 	if (!in_file->f_op || !in_file->f_op->sendfile)
 		goto fput_in;
+	retval = -ESPIPE;
 	if (!ppos)
 		ppos = &in_file->f_pos;
+	else
+		if (!(in_file->f_mode & FMODE_PREAD))
+			goto fput_in;
 	retval = locks_verify_area(FLOCK_VERIFY_READ, in_inode, in_file, *ppos, count);
 	if (retval)
 		goto fput_in;

net/socket.c

@@ -727,9 +727,6 @@ ssize_t sock_sendpage(struct file *file, struct page *page,
 	struct socket *sock;
 	int flags;
 
-	if (ppos != &file->f_pos)
-		return -ESPIPE;
-
 	sock = SOCKET_I(file->f_dentry->d_inode);
 
 	flags = !(file->f_flags & O_NONBLOCK) ? 0 : MSG_DONTWAIT;