Fix the problem (BUG 4964) with unmapped buffers in transaction's
t_sync_data list.  The problem is we need to call filesystem's own
invalidatepage() from block_write_full_page().

block_write_full_page() must call filesystem's invalidatepage().  Otherwise
following nasty race can happen:

   proc 1                                        proc 2
   ------                                        ------
- write some new data to 'offset'
  => bh gets to the transactions data list
                                              - starts truncate
                                                => i_size set to new size
- mpage_writepages()
  - ext3_ordered_writepage() to 'offset'
    - block_write_full_page()
      - page->index > end_index+1
        - block_invalidatepage()
          - discard_buffer()
            - clear_buffer_mapped()

- commit triggers and finds unmapped buffer - BOOM!

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>