btrfs: Cleanup the btrfs_workqueue related function type

The new btrfs_workqueue still use open-coded function defition,
this patch will change them into btrfs_func_t type which is much the
same as kernel workqueue.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>