MINOR: connection: move the cleanup of flag CO_FL_WAIT_ROOM

MINOR: connection: move the cleanup of flag CO_FL_WAIT_ROOM

This flag is only used when reading using splicing for now, and is only
set when a pipe full condition is met, so we can simplify its reset
condition in conn_refresh_polling_flags so that it's cleared at the
same time as the other ones, only when the control layer is ready.

This flag could be used more, to mark that a buffer full condition was
met with any receive method in order to simplify polling management.
This should probably be revisited after 1.8.