Print the first argument of umount2 syscall as a path

* umount.c (SYS_FUNC(umount2)): Use printpath instead of printstr.

This fixes Debian bug #785050.
diff --git a/umount.c b/umount.c
index 8755afc..4f599ea 100644
--- a/umount.c
+++ b/umount.c
@@ -8,7 +8,7 @@
 
 SYS_FUNC(umount2)
 {
-	printstr(tcp, tcp->u_arg[0], -1);
+	printpath(tcp, tcp->u_arg[0]);
 	tprints(", ");
 	printflags(umount_flags, tcp->u_arg[1], "MNT_???");