libxfs: get rid of b_bcount from xfs_buf

libxfs: get rid of b_bcount from xfs_buf

We no longer use it in the kernel - it has been replaced by b_length
and it only exists in userspace because we haven't converted it
over. Do that now before we introduce a heap of code that doesn't
ever set it and so breaks all the progs code.

WHile we are doing this, kill the XFS_BUF_SIZE macro that has also
been removed from the kernel, too.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>