Add parenthesis to btop and ptob macros

Add missing parenthesis around btop and ptob macros to ensure
operation ordering is preserved after expansion.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #660

include/sys/param.h

@@ -28,8 +28,8 @@
 #include <asm/page.h>
 
 /* Pages to bytes and back */
-#define ptob(pages)			(pages << PAGE_SHIFT)
-#define btop(bytes)			(bytes >> PAGE_SHIFT)
+#define ptob(pages)			((pages) << PAGE_SHIFT)
+#define btop(bytes)			((bytes) >> PAGE_SHIFT)
 
 #define MAXUID				UINT32_MAX