kernel - Fix improper assertion panic in vinvalbuf()
* Related to the removal of vhold/vdrop from buffer cache buffers, the
state of a vnode being cleaned can now contain more active buffers and
I/O's at the time of the vinvalbuf() call.
* Remove the 'vinvalbuf: dirty bufs' assertion and panic. It is no longer
a correct assertion. Note that we've also had sporatic reports of this
panic even prior to the work so it might not have been a completely
correct assertion before either.
* Rework the vinvalbuf buffer-flushing and I/O-waiting code a bit. We
have to wait for I/O at least once and it is probably a good idea to
wait for I/O after each buffer flush pass too, to avoid live-locks.
Reported-by: vsrinivas