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/recov.c b/lib/raid6/recov.c
index 2609f00..8590d19 100644
--- a/lib/raid6/recov.c
+++ b/lib/raid6/recov.c
@@ -11,7 +11,7 @@
  * ----------------------------------------------------------------------- */
 
 /*
- * raid6recov.c
+ * raid6/recov.c
  *
  * RAID-6 data recovery in dual failure mode.  In single failure mode,
  * use the RAID-5 algorithm (or, in the case of Q failure, just reconstruct