libceph: add signed type limits

Flesh out the limits defined in <linux/ceph/decode.h> to include the
maximum and minimum values for signed type S8, S16, S32, and S64.
Signed-off-by: NAlex Elder <elder@inktank.com>
Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>

include/linux/ceph/decode.h

@@ -10,10 +10,20 @@
 
 /* This seemed to be the easiest place to define these */
 
-#define	U8_MAX	((u8)  (~0U))
-#define	U16_MAX	((u16) (~0U))
-#define	U32_MAX	((u32) (~0U))
-#define	U64_MAX	((u64) (~0ULL))
+#define	U8_MAX	((u8)(~0U))
+#define	U16_MAX	((u16)(~0U))
+#define	U32_MAX	((u32)(~0U))
+#define	U64_MAX	((u64)(~0ULL))
+
+#define	S8_MAX	((s8)(U8_MAX >> 1))
+#define	S16_MAX	((s16)(U16_MAX >> 1))
+#define	S32_MAX	((s32)(U32_MAX >> 1))
+#define	S64_MAX	((s64)(U64_MAX >> 1LL))
+
+#define	S8_MIN	((s8)(-S8_MAX - 1))
+#define	S16_MIN	((s16)(-S16_MAX - 1))
+#define	S32_MIN	((s32)(-S32_MAX - 1))
+#define	S64_MIN	((s64)(-S64_MAX - 1LL))
 
 /*
  * in all cases,