iscsi_which_events: do not set POLLOUT if context is corked

if the socket is corked we otherwise set POLLOUT and then
do not sent. Depending on the event loop implementation this
can result in a busy wait.

Signed-off-by: Peter Lieven <pl@kamp.de>
Merge pull request #126 from plieven/async_avoid_busy_wait

iscsi_which_events: do not set POLLOUT if context is corked