btrfs-progs: include kerncompat.h in raid6.c, define __attribute_const__

raid6.c was failing to build for Goffredo and me due to
__attribute_const__ being undefined.

Define it in kerncompat.h and include that; this also makes
sure BITS_PER_LONG is defined for raid6.c, prior to this it
was not defined, at least in my build.

Finally, redefine BITS_PER_LONG in a way that it can be
tested in the preprocessor macro.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/raid6.c b/raid6.c
index 3a42bdf..ce0f655 100644
--- a/raid6.c
+++ b/raid6.c
@@ -19,6 +19,8 @@
  */
 #include <stdint.h>
 #include <unistd.h>
+#include "kerncompat.h"
+
 /*
  * This is the C data type to use
  */