Replace lockmgr for vnodes

[Andrew Doran <ad%netbsd.org@localhost>]

Hi,

The following patch uses a reader/writer lock to replace the lockmgr locks
on vnodes. There are very few users of lockmgr() left and this is a step
towards removing it.

        http://www.netbsd.org/~ad/vnlock.diff

On a dual core box there is a slight boost doing make -j8 cleandir on the
source tree.

42.31s real    44.03s user    23.86s system     lockmgr, from cold
33.89s real    43.47s user    23.81s system     lockmgr, warm    

42.15s real    43.65s user    23.83s system     rwlock, from cold
33.68s real    43.46s user    23.40s system     rwlock, warm    

Comments?

Thanks,
Andrew