uml: miscellaneous code cleanups

Code tidying -
	the pid field of struct irq_fd isn't used, so it is removed
     	os_set_fd_async needed to read flags before changing them, it
doesn't need a pid passed in because it can call getpid itself, and a
block of unused code needed deleting
	os_get_exec_close was unused, so it is removed
	ptrace_child called _exit for historical reasons which are no
longer valid, so just calls exit instead

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/os-Linux/start_up.c

@@ -60,7 +60,8 @@ static int ptrace_child(void)
 		 * the UML code itself.
 		 */
 		ret = 2;
-	_exit(ret);
+
+	exit(ret);
 }
 
 static void fatal_perror(const char *str)