Properly vget() vnodes that the syncer intends to VOP_FSYNC(), rather then

Properly vget() vnodes that the syncer intends to VOP_FSYNC(), rather then
just locking the vnode.  The original FreeBSD code allowed a vnode to be
locked without being refd, but DragonFly requires that a vnode be ref'd
if it is going to be locked.  This fixes a rare panic that can occur when
the syncer is flushing a vnode that mainline code tries to access at the
same time.

This also brings the syncer inline with the filesystem sync code which has
been using normal vget()'s for several months now.

Reported-by: David Rhodus