regress_ssl: make ssl/bufferevent_wm_filter more fault-tolerance Due to inplace callbacks (i.e. no BEV_OPT_DEFER_CALLBACKS) we cannot be sure that wm_transfer() will not be called recursively and indeed it still happens sometimes, and the referenced patch increase amount of this times, especially for linux/poll. Fixes: 66304a23cf748714159c988e78f35401c5352827 ("Fix ssl/bufferevent_wm_filter when bev does not reach watermark on break") (cherry picked from commit 54c6fe3c06cb5342130467ecf0f78e3f6d854a36)