Simplify taskqgroup inititialization.

Simplify taskqgroup inititialization.

taskqgroup initialization was broken into two steps:

1. allocate the taskqgroup structure, at SI_SUB_TASKQ;
2. initialize taskqueues, start taskqueue threads, enqueue "binder"
   tasks to bind threads to specific CPUs, at SI_SUB_SMP.

Step 2 tries to handle the case where tasks have already been attached
to a queue, by migrating them to their intended queue.  In particular,
tasks can't be enqueued before step 2 has completed.  This breaks NFS
mountroot on systems using an iflib-based driver when EARLY_AP_STARTUP
is not defined, since mountroot happens before SI_SUB_SMP in this case.

Simplify initialization: do all initialization except for CPU binding at
SI_SUB_TASKQ.  This means that until CPU binding is completed, group
tasks may be executed on a CPU other than that to which they were bound,
but this should not be a problem for existing users of the taskqgroup
KPIs.

Reported by:	sbruno
Tested by:	bdragon, sbruno
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D24188