When the global buffer LRU was present, dirty ext2 indirect blocks were
automatically scheduled for writeback alongside their data.

I added write_mapping_buffers() to replace this - the idea was to
schedule the indirects close in time to the scheduling of their data.

It works OK for small-to-medium sized files but for large, linear writes
it doesn't work: the request queue is completely full of file data and
when we later come to scheduling the indirects, their neighbouring data
has already been written.

So writeback of really huge files tends to be a bit seeky.

So.  Kill it.  Will fix this problem by other means.