- Fix the fix to the fix to the sector_t printing in buffer_io_error()

- A few microoptimisations in buffer.c.  Replace:

	set_buffer_foo(bh);

  with

	if (!buffer_foo(bh))
		set_buffer_foo(bh);

  when buffer_fooness is likely.  To avoid the buslocked rmw, and to
  avoid dirtying a cacheline.


- export write_mapping_buffers() - filesystems which put buffers on
  mapping->private_list need this function for I/O scheduling reasons.