um: Use swap() to make code cleaner

Use the macro 'swap()' defined in 'include/linux/minmax.h' to avoid
opencoding it.
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/sigio.c

@@ -3,6 +3,7 @@
  * Copyright (C) 2002 - 2008 Jeff Dike (jdike@{addtoit,linux.intel}.com)
  */
 
+#include <linux/minmax.h>
 #include <unistd.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -50,7 +51,7 @@ static struct pollfds all_sigio_fds;
 
 static int write_sigio_thread(void *unused)
 {
-	struct pollfds *fds, tmp;
+	struct pollfds *fds;
 	struct pollfd *p;
 	int i, n, respond_fd;
 	char c;
@@ -77,9 +78,7 @@ static int write_sigio_thread(void *unused)
 					       "write_sigio_thread : "
 					       "read on socket failed, "
 					       "err = %d\n", errno);
-				tmp = current_poll;
-				current_poll = next_poll;
-				next_poll = tmp;
+				swap(current_poll, next_poll);
 				respond_fd = sigio_private[1];
 			}
 			else {