Move ic_check_send_space clear to the actual check.

It closes tiny race when the flag could be set between being cleared
and the space is checked, that would create us some more work. The
flag setting is protected by both locks, so we can clear it in either
place, but in between both locks are dropped.

MFC after: 1 week

(cherry picked from commit afc3e54eeee635a525c88e4678cc38e3219302c3)