This is the replacement for write_mapping_buffers().

Whenever the mpage code sees that it has just written a block which had
buffer_boundary() set, it assumes that the next block is dirty
filesystem metadata.  (This is a good assumption - that's what
buffer_boundary is for).

So we do a lookup in the blockdev mapping for the next block and it if
is present and dirty, then schedule it for IO.

So the indirect blocks in the blockdev mapping get merged with the data
blocks in the file mapping.

This is a bit more general than the write_mapping_buffers() approach.
write_mapping_buffers() required that the fs carefully maintain the
correct buffers on the mapping->private_list, and that the fs call
write_mapping_buffers(), and the implementation was generally rather
yuk.

This version will "just work" for filesystems which implement
buffer_boundary correctly.  Currently this is ext2, ext3 and some
not-yet-merged reiserfs patches.  JFS implements buffer_boundary() but
does not use ext2-like layouts - so there will be no change there.

Works nicely.