uml: simplify helper stack handling

run_helper and run_helper_thread had arguments which were the same in all
callers.  run_helper's stack_out was always NULL and run_helper_thread's
stack_order was always 0.  These are now gone, and the constants folded
into the code.

Also fixed leaks of the helper stack in the AIO and SIGIO code.

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>
diff --git a/arch/um/drivers/slirp_user.c b/arch/um/drivers/slirp_user.c
index 39f889f..0e462f6 100644
--- a/arch/um/drivers/slirp_user.c
+++ b/arch/um/drivers/slirp_user.c
@@ -42,7 +42,7 @@
 
 	pe_data.stdin = fd;
 	pe_data.stdout = fd;
-	pid = run_helper(slirp_pre_exec, &pe_data, argv, NULL);
+	pid = run_helper(slirp_pre_exec, &pe_data, argv);
 
 	return(pid);
 }