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>

kerncompat.h

@@ -36,7 +36,7 @@
 #define gfp_t int
 #define get_cpu_var(p) (p)
 #define __get_cpu_var(p) (p)
-#define BITS_PER_LONG (sizeof(long) * 8)
+#define BITS_PER_LONG (__SIZEOF_LONG__ * 8)
 #define __GFP_BITS_SHIFT 20
 #define __GFP_BITS_MASK ((int)((1 << __GFP_BITS_SHIFT) - 1))
 #define GFP_KERNEL 0
@@ -126,6 +126,10 @@ static inline int mutex_is_locked(struct mutex *m)
 #define BITOP_MASK(nr)		(1UL << ((nr) % BITS_PER_LONG))
 #define BITOP_WORD(nr)		((nr) / BITS_PER_LONG)
 
+#ifndef __attribute_const__
+#define __attribute_const__	__attribute__((__const__))
+#endif
+
 /**
  * __set_bit - Set a bit in memory
  * @nr: the bit to set

raid6.c

@@ -19,6 +19,8 @@
  */
 #include <stdint.h>
 #include <unistd.h>
+#include "kerncompat.h"
+
 /*
  * This is the C data type to use
  */