This fixes "change blocks numbers on the fly" in case when "prepare
write page" is in the call chain, in this case some buffers may be not
uptodate and not mapped, we should care to map them and load from disk.

This patch was tested with:
 - ufs regressions simple tests
 - fsx-linux
 - ltp(20060306)
 - untar and build kernel
Signed-off-by: NEvgeniy Dushistov <dushistov@mail.ru>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>