xfs: fix shadow warning in xfs_da3_root_split()

Use icnodehdr for struct xfs_da3_icnode_hdr instead of nodehdr
(already declared above).

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>