Patch from Chris Mason <mason@suse.com>

The patch below is against 2.5.59, various forms have been floating
around for a while, and Andrew recently included this fixed version in
2.5.55-mm.  The end result is faster reads and writes for reiserfs.

This adds reiserfs support for readpages, along with a support func in
fs/mpage.c to deal with the reiserfs_get_block call sending back up to
date buffers with packed tails copied into them.

Most of the changes are to reiserfs_writepage, which still had many
2.4isms in the way it started io, dealt with errors and handled the bh
state bits.  I've also added an optimization so it only starts
transactions when we need to copy a packed tail into the btree or fill a
hole, instead of any time reiserfs_writepage hits an unmapped buffer.