IB/ipath: Fix hang on module unload

Handle the case where posting a send is requested when the link is
down.  This fixes <https://bugs.openfabrics.org/show_bug.cgi?id=1117>.

Signed-off-by: Yannick Cote <yannick.cote@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>