md: Factor out RAID6 algorithms into lib/

We'll want to use these in btrfs too.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/lib/Kconfig b/lib/Kconfig
index bb1326d..1cc756c 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -7,6 +7,9 @@
 
 menu "Library routines"
 
+config RAID6_PQ
+	tristate
+
 config BITREVERSE
 	tristate