Update SWAP_FLAG_* constants

* swapon.c: Ensure that SWAP_FLAG_DISCARD_ONCE and
SWAP_FLAG_DISCARD_PAGES are defined.
* xlat/swap_flags.in: Add SWAP_FLAG_DISCARD_ONCE and
SWAP_FLAG_DISCARD_PAGES.
diff --git a/swapon.c b/swapon.c
index c26bb06..8200a9b 100644
--- a/swapon.c
+++ b/swapon.c
@@ -8,6 +8,12 @@
 #ifndef SWAP_FLAG_DISCARD
 # define SWAP_FLAG_DISCARD 0x10000
 #endif
+#ifndef SWAP_FLAG_DISCARD_ONCE
+# define SWAP_FLAG_DISCARD_ONCE 0x20000
+#endif
+#ifndef SWAP_FLAG_DISCARD_PAGES
+# define SWAP_FLAG_DISCARD_PAGES 0x40000
+#endif
 
 #include "xlat/swap_flags.h"