sync_vnode() has some messy code about locking in order to deal with

sync_vnode() has some messy code about locking in order to deal with
mount fs needing Giant to be held when processing bufobjs.
Use a different subqueue for pending workitems on filesystems requiring
Giant. This simplifies the code notably and also reduces the number of
Giant acquisitions (and the whole processing cost).

Suggested by:	jeff
Reviewed by:	kib
Tested by:	pho