b=2030

b=2030
Queue IO in writepage when dirty pages first arrive there.  Previously
it was assumed that commit_write() was involved, which isn't the case
for writes via mmap().  mmap() still isn't correct across the cluster,
but at least pages won't get lost in writeback.

In the process of adding a sanity test for this some mmap() operations
are added to multiop and sanity gets a helper to find the page size.