PG_BUSY and read locks in the VM system

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

The recent change to make vmobjlock & friends into RW locks is still a bit
useless because while PG_BUSY can be observed or held off while holding a
RW_READER lock, nothing can be done if it's set.  To fix that I'd like to
change it so that the WANTED flag is stored in pg->pqflags and covered by
pg->interlock.

From there it's not huge strech to imagine how you might do basic page fault
handing for already in-core pages while holding only a read lock.

http://www.netbsd.org/~ad/2020/wanted.diff

Comments?

Thanks,
Andrew