Prepare for kernel xfs_buf  alignment by getting rid of the
xfs_buf_t typedef from userspace.

[darrick: This patch is a port of a userspace patch removing the
xfs_buf_t typedef in preparation to make the userspace xfs_buf code
behave more like its kernel counterpart.]
Signed-off-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NDave Chinner <dchinner@redhat.com>