Do not exit ctl_be_block_worker() prematurely.

Return while there are any I/Os in a queue may result in them stuck
indefinitely, since there is only one taskqueue task for all of them.
I think I've reproduced this by switching ha_role to secondary under
heavy load.

MFC after: 3 days

(cherry picked from commit 6ed39db2573bb808ac2c206cd6c831f0be86219c)