reiserfs_file_write unlocks the pages it operated on before updating
i_size.  This can lead to races with writepage, who checks i_size when
deciding how much of the file to zero out.

This patch also replaces SetPageReferenced with mark_page_accessed() in
reiserfs_file_write

This was verified to fix the BitKeeper data corruption problems that
Steven Cole has been debugging, where concurrent writes to a file and
writebacks to disk would cause zeroes in the file when CONFIG_PREEMPT
was enabled.