IB/mlx4: Make sure inline data segments don't cross a 64 byte boundary

Inline data segments in send WQEs are not allowed to cross a 64 byte
boundary.  We use inline data segments to hold the UD headers for MLX
QPs (QP0 and QP1).  A send with GRH on QP1 will have a UD header that
is too big to fit in a single inline data segment without crossing a
64 byte boundary, so split the header into two inline data segments.

Signed-off-by: Roland Dreier <rolandd@cisco.com>