IPoIB/cm: Set tx_wr.num_sge in connected mode post_send()

Commit 7143740d

 ("IPoIB: Add send gather support") made it possible
for tx_wr.num_sge to be != 1 -- this happens if send gather support is
enabled.  However, the code in the connected mode post_send() function
assumes the old invariant, namely that tx_wr.num_sge is always 1.  Fix
this by explicitly setting tx_wr.num_sge to 1 in the CM post_send().

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