IB/mlx4: Factor out setting WQE data segment entries

Factor code to set data segment entries out of mlx4_ib_post_send()
into set_data_seg().  This cleans up the code and lets the compiler do
a better job -- on x86_64:

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-16 (-16)
function                                     old     new   delta
mlx4_ib_post_send                           1598    1582     -16

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