Removal of PG_launder.

It's not obvious (to me) why this ever existed.  If it's to prevent
deadlocks then I'd like to know who was performing __GFP_FS allocations
while holding a page lock?

But in 2.5, the only memory allocations which are performed when the
caller holds PG_writeback against an unsubmitted page are those which
occur inside submit_bh().  There will be no __GFS_FS allocations in
that call chain.

Removing PG_launder means that memory allocators can block on any
PageWriteback() page at all, which reduces the risk of very long list
walks inside pagemap_lru_lock in shrink_cache().