um: winch_interrupt() can happen inside of free_winch()

...  so set winch->fd to -1 before doing free_irq(), to avoid having
winch_interrupt() come from/during the latter and attempt to do
reactivate_fd() on something that's already gone.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/line.c

@@ -725,6 +725,8 @@ struct winch {
 
 static void free_winch(struct winch *winch, int free_irq_ok)
 {
+	int fd = winch->fd;
+	winch->fd = -1;
 	if (free_irq_ok)
 		free_irq(WINCH_IRQ, winch);
 
@@ -732,8 +734,8 @@ static void free_winch(struct winch *winch, int free_irq_ok)
 
 	if (winch->pid != -1)
 		os_kill_process(winch->pid, 1);
-	if (winch->fd != -1)
-		os_close_file(winch->fd);
+	if (fd != -1)
+		os_close_file(fd);
 	if (winch->stack != 0)
 		free_stack(winch->stack, 0);
 	kfree(winch);