IB/mlx4: Use __set_data_seg() in mlx4_ib_post_recv()

Use a __set_data_seg() helper in mlx4_ib_post_recv() too; in addition
to making the code easier to read, this also allows gcc to generate
better code -- on x86_64:

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-8 (-8)
function                                     old     new   delta
mlx4_ib_post_recv                            359     351      -8

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