When shutting down the syncer kernel thread, first tell it to run

When shutting down the syncer kernel thread, first tell it to run
faster and iterate to over its work list a few times in an attempt
to empty the work list before the syncer terminates.  This leaves
fewer dirty blocks to be written at the "syncing disks" stage and
keeps the the "giving up on N buffers" problem from being triggered
by the presence of a large soft updates work list at system shutdown
time.  The downside is that the syncer takes noticeably longer to
terminate.

Tested by:	"Arjan van Leeuwen" <avleeuwen AT piwebs DOT com>
Approved by:	mckusick