IB/mthca: Factor out setting WQE data segment entries

IB/mthca: Factor out setting WQE data segment entries

Factor code to set data segment entries out of the work request
posting functions into inline functions mthca_set_data_seg() and
mthca_set_data_seg_inval().  This makes the code more readable and
also allows the compiler to do a better job -- on x86_64:

add/remove: 0/0 grow/shrink: 0/6 up/down: 0/-69 (-69)
function                                     old     new   delta
mthca_arbel_post_srq_recv                    373     369      -4
mthca_arbel_post_receive                     570     562      -8
mthca_tavor_post_srq_recv                    520     508     -12
mthca_tavor_post_send                       1344    1330     -14
mthca_arbel_post_send                       1481    1467     -14
mthca_tavor_post_receive                     792     775     -17

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