Further tidyup of raid6 naming in lib/raid6

Rename raid6/raid6x86.h to raid6/x86.h
and modify some comments.

Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
index df7ff727..b595f56 100644
--- a/lib/raid6/algos.c
+++ b/lib/raid6/algos.c
@@ -11,7 +11,7 @@
  * ----------------------------------------------------------------------- */
 
 /*
- * raid6algos.c
+ * raid6/algos.c
  *
  * Algorithm list and algorithm selection for RAID-6
  */