sysmacros: Make P2ROUNDUP not trigger int overflow

The original P2ROUNDUP and P2ROUNDUP_TYPED macros contain -x which
triggers PaX's integer overflow detection for unsigned integers.
Replace the macros with an equivalent version that does not trigger
the overflow.

Axioms:
A. (-(x)) === (~((x) - 1)) === (~(x) + 1) under two's complement.
B. ~(x & y) === ((~(x)) | (~(y))) under De Morgan's law.
C. ~(~x) === x under the law of excluded middle.

Proof:
0. (-(-(x) & -(align))) original
1. (~(-(x) & -(align)) + 1) by A
2. (((~(-(x))) | (~(-(align)))) + 1) by B
3. (((~(~((x) - 1))) | (~(~((align) - 1)))) + 1) by A
4. (((((x) - 1)) | (((align) - 1))) + 1) by C
Q.E.D.

Signed-off-by: Jason Zaman <[email protected]>
Reviewed-by: Chris Dunlop <[email protected]>
Reviewed-by: Richard Yao <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #3949

lib/libspl/include/sys/sysmacros.h

@@ -49,9 +49,7 @@
  */
 #define	P2ALIGN(x, align)	((x) & -(align))
 #define	P2CROSS(x, y, align)	(((x) ^ (y)) > (align) - 1)
-#define	P2ROUNDUP(x, align)	(-(-(x) & -(align)))
-#define	P2ROUNDUP_TYPED(x, align, type) \
-				(-(-(type)(x) & -(type)(align)))
+#define	P2ROUNDUP(x, align)	((((x) - 1) | ((align) - 1)) + 1)
 #define	P2BOUNDARY(off, len, align) \
 				(((off) ^ ((off) + (len) - 1)) > (align) - 1)
 #define	P2PHASE(x, align)	((x) & ((align) - 1))
@@ -79,7 +77,7 @@
 #define	P2NPHASE_TYPED(x, align, type)		\
 	(-(type)(x) & ((type)(align) - 1))
 #define	P2ROUNDUP_TYPED(x, align, type)		\
-	(-(-(type)(x) & -(type)(align)))
+	((((type)(x) - 1) | ((type)(align) - 1)) + 1)
 #define	P2END_TYPED(x, align, type)		\
 	(-(~(type)(x) & -(type)(align)))
 #define	P2PHASEUP_TYPED(x, align, phase, type)	\