add SWAP_FLAG_DISCARD to sys/swap.h

include/sys/swap.h

@@ -9,6 +9,7 @@ extern "C" {
 #define	SWAP_FLAG_PREFER        0x8000
 #define	SWAP_FLAG_PRIO_MASK     0x7fff
 #define	SWAP_FLAG_PRIO_SHIFT    0
+#define SWAP_FLAG_DISCARD       0x10000
 
 int swapon (const char *, int);
 int swapoff (const char *);