FreeBSD: Improve taskq wrapper

• Group tqent_task and tqent_timeout_task into a union. They are

never used same time. This shrinks taskq_ent_t from 192 to 160 bytes.

• Remove tqent_registered. Use tqent_id != 0 instead.
• Remove tqent_cancelled. Use taskqueue pending counter instead.
• Change tqent_type into uint_t. We don't need to pack it any more.
• Change tqent_rc into uint_t, matching refcount(9).
• Take shared locks in taskq_lookup().
• Call proper taskqueue_drain_timeout() for TIMEOUT_TASK in

taskq_cancel_id() and taskq_wait_id().

• Switch from CK_LIST to regular LIST.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15356