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

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>