workqueue: make workqueue->name[] fixed len

Currently workqueue->name[] is of flexible length.  We want to use the
flexible field for something more useful and there isn't much benefit
in allowing arbitrary name length anyway.  Make it fixed len capping
at 24 bytes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>